Diffractive optical modulator and method for producing the same, infrared sensor including such a diffractive optical modulator and method for producing the same, and display device including such a diffractive optical modulator

ABSTRACT

The diffractive optical modulator of the invention includes: a plate having a portion functioning as a first electrode; a spacer layer formed on the plate; and a grating consisting of a plurality of beams having a portion functioning as a second electrode, both ends of the beams being supported on the spacer layer. In the diffractive optical modulator, by adjusting a voltage applied between the first electrode and the second electrode, a distance between the beams and the plate is varied, thereby controlling the diffraction efficiency. An insulating layer is further provided between the plate and the plurality of beams.

This is a division of copending application Ser. No. 08/492,894, filedJun. 20, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffractive optical modulator formodulating a light intensity and a method for producing the same; aninfrared sensor including such a diffractive optical modulator and amethod for producing the same; and a display device including such adiffractive optical modulator.

2. Description of the Related Art

A diffractive optical modulator modulates the intensity of incominglight by using the diffraction. A conventional diffractive opticalmodulator is disclosed, for example, in O. Solgaard et al., "DeformableGrating Optical Modulator," Optics Letters, Vol. 17, No. 9, May 1, 1992.The diffractive optical modulator disclosed in this article modulatesthe light intensity by using the diffraction effects of light. Thismodulator can be produced with a reduced size by an IC process, makingsuch a modulator suitable for mass-producing.

The diffractive optical modulator disclosed in this article includes: asilicon substrate; a spacer (or silicon oxide film) formed in aperipheral region on the silicon substrate; and a grating consisting ofa plurality of fine dielectric beams (or silicon-rich silicon nitridefilms), both ends of which are supported by the spacer. A reflectionfilm functioning also as an electrode is provided on the upper surfaceof the grating. When a voltage is applied between the reflection filmand the silicon substrate, an electrostatic force (or a Coulomb's force)is generated therebetween, so that the grating is deflected. As aresult, the lower surface of the deflected grating comes into contactwith the upper surface of the silicon substrate. Since the distancebetween the reflection film provided on the upper surface of the gratingand the reflection film provided on the upper surface of the siliconsubstrate is varied in accordance with the application of a voltage, thediffraction efficiency is also varied.

In a conventional diffractive optical modulator, the beams are made of adielectric material, and the modulator is driven by applying a voltagebetween the reflection film provided on the upper surface of the beamsand the substrate. Accordingly, in the case of modulating light having along wavelength such as infrared light, the distance between the upperand the lower electrodes becomes long, so that the driving voltage isrequired to be adversely high. In addition, the beams are formed bynitride films, and therefore a strong residual tensile stress is causedin the films. In general, the residual stress in a nitride film isalmost as large as 1 GPa. In the above-mentioned conventional example,the beams are formed by silicon-rich nitride films, so that the residualstress is reduced to about 200 MPa. However, it is very difficult toreduce the tensile stress to a small one in accordance with such amethod for reducing the stress, and the uniformity of the film is alsodegraded. Moreover, the spacer layer is formed by a silicon oxide film,and then removed by a wet etching (W/E) process. When the grating isdried after being rinsed, the surface tension of the rinsing liquidadversely causes the sticking of the beams onto the surface of thesubstrate. In order to solve such problems, a method in whichprotrusions provided on the lower surfaces of the beams prevent thesticking, or a so-called freeze drying method in which the beams arefrozen, for example, in pure water after being rinsed and the frozenpure water is sublimated under vacuum has been used. However, both themethods disadvantageously complicate the production process.

SUMMARY OF THE INVENTION

The diffractive optical modulator of the invention includes: a platehaving a portion functioning as a first electrode; a spacer layer formedon the plate; and a grating consisting of a plurality of beams having aportion functioning as a second electrode, both ends of the beams beingsupported on the spacer layer. In the diffractive optical modulator, byadjusting a voltage applied between the first electrode and the secondelectrode, a distance between the beams and the plate is varied, therebycontrolling the diffraction efficiency, and an insulating layer isfurther provided between the plate and the plurality of beams.

In one embodiment, a reflection film is formed on a surface of theinsulating layer and on surfaces of the beams.

In another embodiment, the plate is made of a semiconductor functioningas the first electrode.

In still another embodiment, the plate consists of a conductive layerfunctioning as the first electrode and an insulating substrate forsupporting the conductive layer.

In still another embodiment, at least lower surfaces of the beams aremade of a conductive material.

In still another embodiment, at least lower surfaces of the beams aremade of a conductive material which is not likely to be oxidized.

In still another embodiment, the spacer layer is made of an organicmaterial.

In still another embodiment, the conductive material is selected fromthe group consisting of Au, Pt, Ti, an NiCr alloy, a CuNi alloy, chromesteel, and a conductive organic material.

In still another embodiment, the spacer layer is made of the samematerial as a material of the plurality of beams.

In still another embodiment, a width of the beams supported on thespacer layer in a longitudinal direction is less than twice of athickness of the beams.

The diffractive optical modulator according to another aspect of theinvention includes: a plate having a portion functioning as a firstelectrode, and an upper surface and a bottom surface; a spacer layerformed on the upper surface of plate; and a grating consisting of aplurality of beams having a portion functioning as a second electrode,both ends of the beams being supported on the spacer layer. In thediffractive optical modulator, by adjusting a voltage applied betweenthe first electrode and the second electrode, a distance between thebeams and the plate is varied, thereby controlling the diffractionefficiency, and a first antireflection film made of an insulatingmaterial is further provided on the upper surface of the plate, and asecond antireflection film made of an insulating material is furtherprovided on the bottom surface of the plate, and each of the beamsconsists of a beam-shaped reflection film functioning as the secondelectrode and being made of a conductive material, and an elastic layerformed on the beam-shaped reflection film.

The diffractive optical modulator according to still another aspect ofthe invention includes: a plate having a portion functioning as a firstelectrode; a spacer layer formed on the plate; and a grating consistingof a plurality of beams having a portion functioning as a secondelectrode, both ends of the beams being supported on the spacer layer.In the diffractive optical modulator, by adjusting a voltage appliedbetween the first electrode and the second electrode, a distance betweenthe beams and the plate is varied, thereby controlling the diffractionefficiency, and the plurality of beams are arranged so that a movabledistance between the plurality of beams and the plate becomes minimum onan optical axis of incoming light.

The diffractive optical modulator according to still another aspect ofthe invention includes: a plate having a portion functioning as a firstelectrode; a spacer layer formed on the plate; and a grating consistingof a plurality of beams having a portion functioning as a secondelectrode, both ends of the beams being supported on the spacer layer.In the diffractive optical modulator, by adjusting a voltage appliedbetween the first electrode and the second electrode, a distance betweenthe beams and the plate is varied, thereby controlling the diffractionefficiency, and a thickness of the plurality of beams is adjusted so asto be minimal on an optical axis of incoming light.

In one embodiment, the first electrode is grounded, and a voltage isapplied to the beam-shaped reflection film.

In another embodiment, the elastic layer is made of the same material asa material of the beam-shaped reflection film.

According to still another aspect of the invention, a method forproducing the diffractive optical modulator is provided. The methodincludes the steps of: depositing a first layer functioning as a spacerlayer on a plate; and depositing a second layer functioning as beams onthe spacer layer. In this method, during the step of depositing thefirst layer, the first layer is deposited while moving a shield disposedbetween a deposition source for supplying a material of the first layertowards the plate and the plate, thereby varying a thickness of thefirst layer at respective positions.

According to still another aspect of the invention, a method forproducing the diffractive optical modulator is provided. The methodincludes the steps of: depositing a first layer functioning as a spacerlayer on a plate; and depositing a second layer functioning as beams onthe spacer layer. In this method, during the step of depositing thesecond layer, the second layer is deposited while moving a shielddisposed between a deposition source for supplying a material of thesecond layer towards the plate and the plate, thereby varying athickness of the second layer at respective positions.

According to still another aspect of the invention, a method forproducing the diffractive optical modulator is provided. The methodincludes the steps of: forming an insulating film on a plate having aportion functioning as a first electrode; depositing an organic film onthe insulating film; depositing a conductive thin film on the organicfilm; patterning the conductive thin film, thereby forming a pluralityof beams functioning as a second electrode; and removing a predeterminedportion of the organic film by a dry etching process, thereby forming aspacer for supporting both ends of the plurality of beams.

According to still another aspect of the invention, a method for drivingthe diffractive optical modulator is provided. In this method, voltagesin a rectangular waveform having an equal absolute value and oppositepolarities are applied to the first electrode and the second electrode,respectively.

According to still another aspect of the invention, an infrared sensorincluding: a lens for converging infrared light and a pyroelectricelement is provided. In the infrared sensor, a diffractive opticalmodulator for receiving the infrared light converged by the lens and foroutputting at least a part of the infrared light toward the pyroelectricelement is further provided. The diffractive optical modulator includes:a plate having a portion functioning as a first electrode; a spacerlayer formed on the plate; and a grating consisting of a plurality ofbeams having a portion functioning as a second electrode, both ends ofthe beams being supported on the spacer layer. In the infrared sensor,by adjusting a voltage applied between the first electrode and thesecond electrode, a distance between the beams and the plate is varied,thereby controlling the diffraction efficiency of the diffractiveoptical modulator.

In one embodiment, the infrared sensor further includes: a signalamplifier, connected to the pyroelectric element, for outputting anelectric signal indicating an amount of infrared light received by thepyroelectric element; and a plurality of electrode pins connected to thefirst and the second electrodes of the diffractive optical modulator andthe signal amplifier and the pyroelectric element, the electrode pinsexternally protruding from the bottom surface of the seal case.

In another embodiment, the infrared sensor further includes a supportingplate for supporting the pyroelectric element and the signal amplifier.

In still another embodiment, at least one of the plurality of electrodepins extends to an inside of the seal case, and the at least oneelectrode pin supports the supporting plate.

In still another embodiment, the infrared sensor further includes ashield which is disposed between the pyroelectric element and thediffractive optical modulator and is grounded.

In still another embodiment, the diffractive optical modulator has aninclination angle (θ_(t)) of 45 degrees or less with respect to an uppersurface of the seal case.

In still another embodiment, the angle (θ_(t)) is 25 degrees or less.

In still another embodiment, the plate is disposed being inclined sothat a normal with respect to a principal plane of the plate is notparallel to an optical axis of the lens.

In still another embodiment, the diffractive optical modulator isdisposed so that only zero-order diffracted light of light diffracted bythe grating is incident on the pyroelectric element and that thediffracted light other than the zero-order diffracted light is notincident on the pyroelectric element.

In still another embodiment, an amount of the zero-order diffractedlight is varied in accordance with a variation of a distance between thebeams and the plate of the diffractive optical modulator.

In still another embodiment, a seal case having an opening includes thediffractive optical modulator and the pyroelectric element.

In still another embodiment, the lens for converging the infrared lightis provided so as to cover the opening of the seal case.

In still another embodiment, the seal case includes: an upper surfacefor supporting the lens; a bottom surface parallel to the upper surface;and an inclined member for supporting the diffractive optical modulatorso that the diffractive optical modulator is inclined with respect tothe bottom surface by an inclination angle θ_(t), and the diffractiveoptical modulator is disposed on the inclined member.

In still another embodiment, the lens for converging the infrared lightis a diffractive lens.

In still another embodiment, the lens for converging the infrared lighthas a corrugated structure corresponding to a phase modulation of thelens and is made of a material selected from the group consisting of Si,Ge, GaAs, InP, GaP, ZnSe and ZnS.

In still another embodiment, a period of the grating is seven times ormore of a wavelength of the infrared light.

In still another embodiment, the plurality of beams are arranged so thata movable distance of the grating becomes minimum on an optical axis ofincoming infrared light.

In still another embodiment, a thickness of the plurality of beams isadjusted so as to be minimum on an optical axis of incoming infraredlight.

In still another embodiment, the diffractive optical modulator isdisposed so that a direction parallel to a principal plane of the plateand vertical to the beams is vertical to an optical axis of the lens.

In still another embodiment, a movable distance of the beams is set tobe λ(4 cos θ), where λ is a wavelength of the infrared light, and θ isan angle formed between a normal with respect to the principal plane ofthe plate of the diffractive optical modulator and the optical axis ofthe lens.

In still another embodiment, a thickness of the beams is set to be λ/(4cos θ), where λ is a wavelength of the infrared light, and θ is an angleformed between a normal with respect to the principal plane of the plateof the diffractive optical modulator and the optical axis of the lens.

In still another embodiment, an insulating layer is provided between theplate of the diffractive optical modulator and the beams.

In still another embodiment, the beams of the diffractive opticalmodulator are made of a conductive material.

The infrared sensor according to still another aspect of the inventionincludes: a diffractive optical modulator for outputting at least a partof incoming infrared light; a lens; and a pyroelectric element. In theinfrared sensor, the lens converges the infrared light output from thediffractive optical modulator on the pyroelectric element, and thediffractive optical modulator includes: a plate having a portionfunctioning as a first electrode; a spacer layer formed on the plate;and a grating consisting of a plurality of beams having a portionfunctioning as a second electrode, both ends of the beams beingsupported on the spacer layer. In the infrared sensor, by adjusting avoltage applied between the first electrode and the second electrode, adistance between the beams and the plate is varied, thereby controllingthe diffraction efficiency of the diffractive optical modulator.

In one embodiment, the infrared sensor further includes: a signalamplifier, connected to the pyroelectric element, for outputting anelectric signal indicating an amount of infrared light received by thepyroelectric element; and a plurality of electrode pins connected to thefirst and the second electrodes of the diffractive optical modulator andthe signal amplifier and the pyroelectric element, the electrode pinsexternally protruding from the bottom surface of the seal case.

In another embodiment, a seal case having an opening includes thediffractive optical modulator, the pyroelectric element and the lens.

In still another embodiment, an infrared wavelength filter is providedso as to cover the opening of the seal case.

In still another embodiment, the infrared sensor further includes anopening control means provided for the opening.

According to still another aspect of the invention, a method forproducing an infrared sensor is provided. The infrared sensor includes alens for converging infrared light, a pyroelectric element and adiffractive optical modulator for receiving the infrared light convergedby the lens and for outputting at least a part of the infrared light tothe pyroelectric element. A method for producing the diffractive opticalmodulator includes the steps of: depositing a first layer functioning asa spacer layer on a plate; and depositing a second layer functioning asbeams on the spacer layer. In this method, during the step of depositingthe first layer, the first layer is deposited while moving a shielddisposed between a deposition source for supplying a material of thefirst layer towards the plate and the plate, thereby varying a thicknessof the first layer at respective positions.

According to still another aspect of the invention, a method forproducing an infrared sensor is provided. The infrared sensor includes alens for converging infrared light, a pyroelectric element and adiffractive optical modulator for receiving the infrared light convergedby the lens and for outputting at least a part of the infrared light tothe pyroelectric element. A method for producing the diffractive opticalmodulator includes the steps of: depositing a first layer functioning asa spacer layer on a plate; and depositing a second layer functioning asbeams on the spacer layer. In this method, during the step of depositingthe second layer, the second layer is deposited while moving a shielddisposed between a deposition source for supplying a material of thesecond layer towards the plate and the plate, thereby varying athickness of the second layer at respective positions.

According to still another aspect of the invention, a method forproducing an infrared sensor is provided. The infrared sensor includes alens for converging infrared light, a pyroelectric element and adiffractive optical modulator for receiving the infrared light convergedby the lens and for outputting at least a part of the infrared light tothe pyroelectric element. A method for producing the diffractive opticalmodulator includes the steps of: forming an insulating film on a platehaving a portion functioning as a first electrode; depositing an organicfilm on the insulating film; depositing a conductive thin film on theorganic film; patterning the conductive thin film, thereby forming aplurality of beams functioning as a second electrode; and removing apredetermined portion of the organic film by a dry etching process,thereby forming a spacer for supporting both ends of the plurality ofbeams.

According to still another aspect of the invention, a display device isprovided. The display device includes: a light source; a diffractiveoptical modulation unit provided on an optical path of light emittedfrom the light source; and an optical element for imaging light outputfrom the diffractive optical modulation unit. The diffractive opticalmodulation unit is provided with a diffractive grating means, therebycontrolling a diffraction efficiency of the diffractive grating means.

In one embodiment, the diffractive grating means is a reflective typemeans.

In another embodiment, a lattice pitch of the diffractive grating meansis seven times or more of a central value of a waveband of the light.

In still another embodiment, the diffractive optical modulation unitincludes a plurality of diffractive optical modulators two-dimensionallyarranged as the diffractive grating means, and the plurality ofdiffractive optical modulators respectively correspond to a plurality ofpixels. Each of the plurality of diffractive optical modulatorsincludes: a plate having a portion functioning as a first electrode; aspacer layer formed on the plate; and a grating consisting of aplurality of beams having a portion functioning as a second electrode,both ends of the beams being supported on the spacer layer. Thediffractive optical modulator controls the diffraction efficiency byvarying a gap between the beams and the plate by adjusting a voltageapplied between the first electrode and the second electrode.

In still another embodiment, the plurality of diffractive opticalmodulators further include an insulating layer formed between the plateand the plurality of beams.

In still another embodiment, a region for forming a phase differencewhich is one half of a wavelength of the light is provided betweenadjacent modulators of the plurality of diffractive optical modulators.

In still another embodiment, the display device further includes aseparation means for separating the light emitted from the light sourceinto a plurality of light beams having different wavebands. Thediffractive optical modulation unit is disposed on an optical path ofeach of the plurality of light beams.

In still another embodiment, the diffractive optical modulation unitincludes a plurality of diffractive optical modulators two-dimensionallyarranged as the diffractive grating means, and the plurality ofdiffractive optical modulators respectively correspond to a plurality ofpixels. Each of the plurality of diffractive optical modulatorsincludes: a plate having a portion functioning as a first electrode; asupporting beam formed on the plate; and a grating consisting of aplurality of beams having a portion functioning as a second electrode,both ends of the beams being supported on the supporting beam. A widthof the supporting beam is smaller than a width of a movable portion ofeach of the plurality of beams. The diffractive optical modulatorcontrols a diffraction efficiency by varying a gap between the beams andthe plate by adjusting a voltage applied between the first electrode andthe second electrode.

Thus, the invention described herein makes possible the advantages of(1) providing a diffractive optical modulator which can be producedeasily in a small size, which can be driven at a low voltage, and whichexhibits excellent durability and response characteristics, and a methodfor producing the same; (2) providing an infrared sensor including sucha diffractive optical modulator and a method for producing the same; and(3) providing a display device including such a diffractive opticalmodulator.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a fundamental configuration ofan infrared sensor according to a first example of the invention.

FIG. 2 is a plan view seen from above the seal case showing a positionalrelationship between a diffractive optical modulator and a pyroelectricelement according to the first example of the invention.

FIG. 3A is a cross-sectional view of the diffractive optical modulatorof the first example in a state where no voltage is applied, while

FIG. 3B is a cross-sectional view of the diffractive optical modulatorof the first example in a state where a voltage is applied.

FIG. 4 is a graph showing a relationship between the diffractionefficiency and the amount of the phase shift in the diffractive opticalmodulator for the infrared sensor of the first example of the invention.

FIG. 5 is a cross-sectional view showing a fundamental configuration ofa diffractive optical modulator for an infrared sensor according to asecond example of the invention.

FIG. 6 is a cross-sectional view schematically showing a thin filmdeposition process in a production method of the second example.

FIG. 7A is a plan view of an infrared sensor according to a thirdexample of the invention, while

FIG. 7B is a cross-sectional view taken along the line A-A' in FIG. 7A.

FIGS. 8A to 8G are cross-sectional views showing the process steps forproducing the diffractive optical modulator of the third example of theinvention.

FIG. 9A is a cross-sectional view of the diffractive optical modulatorof the third example in a state where no voltage is applied, while

FIG. 9B is a cross-sectional view of the diffractive optical modulatorof the third example in a state where a voltage is applied.

FIG. 10 is a cross-sectional view of a diffractive optical modulator foran infrared sensor according to a fourth example of the invention.

FIGS. 11A to 11I are cross-sectional views showing the process steps forproducing the diffractive optical modulator of the fourth example of theinvention.

FIG. 12A is a cross-sectional view of the diffractive optical modulatorof the fourth example in a state where no voltage is applied, while

FIG. 12B is a cross-sectional view of the diffractive optical modulatorof the fourth example in a state where a voltage is applied.

FIG. 13 is a cross-sectional view of a diffractive optical modulator foran infrared sensor according to a fifth example of the invention.

FIGS. 14A to 14G are cross-sectional views showing the process steps forproducing the diffractive optical modulator of the fifth example of theinvention.

FIGS. 15A to 15C show exemplary waveforms of the driving voltage to beapplied to the diffractive optical modulator of the invention.

FIG. 16 is a perspective view showing a configuration and an arrangementfor an infrared sensor according to a sixth example of the invention.

FIG. 17 is a perspective view showing a fundamental configuration of thedevices disposed on the supporting plate of the infrared sensoraccording to the sixth example of the invention.

FIG. 18 is a side view showing a fundamental configuration of aninfrared sensor according to the sixth example of the invention.

FIG. 19A is a plan view of the diffractive optical modulator accordingto the sixth example of the invention, while

FIG. 19B is a cross-sectional view taken along the line A-A' in FIG.19A.

FIGS. 20A to 20F are cross-sectional views showing the process steps forproducing the diffractive optical modulator of the sixth example of theinvention.

FIG. 21A is a cross-sectional view illustrating the operation of thediffractive optical modulator of the sixth example when the voltage isoff, while

FIG. 21B is a cross-sectional view illustrating the operation of thediffractive optical modulator of the sixth example when the voltage ison.

FIG. 22A is a graph showing the relationship between the inclinationangle and the optimal thickness of a beam in the diffractive opticalmodulator of the sixth example, while

FIG. 22B is a graph showing the relationship between the inclinationangle and the increase in the driving voltage.

FIG. 23A is a cross-sectional view showing the state of the chargesresulting from the application of a voltage in the diffractive opticalmodulator of the sixth example, whose beams have the oxidized lowersurfaces.

FIG. 23B illustrates the behavior of the electrons in the vicinity ofthe contact portion thereof.

FIG. 23C illustrates the state of the charges in the vicinity of thecontact portion thereof resulting from the removal of the appliedvoltage.

FIG. 24 is a graph showing the variation of an exemplary waveform of thevoltage to be applied for driving a diffractive optical modulator whileforcibly removing the residual charges thereof.

FIG. 25A is a cross-sectional view illustrating the behavior of thecharges in the diffractive optical modulator of the sixth example whenthe voltage is on, while

FIG. 25B is a cross-sectional view illustrating the behavior of thecharges in the diffractive optical modulator of the sixth example whenthe voltage is off.

FIG. 26 is a side view showing a fundamental configuration of aninfrared sensor according to a seventh example of the invention.

FIG. 27A is a plan view of the diffractive optical modulator accordingto an eighth example of the invention, while

FIG. 27B is a cross-sectional view taken along the line A-A' in FIG.27A.

FIGS. 28A to 28E are cross-sectional views taken along the line A-A' inFIG. 27A showing the process steps for producing the diffractive opticalmodulator of the eighth example of the invention.

FIGS. 29A to 29E are cross-sectional views taken along the line B-B' inFIG. 27A showing the process steps for producing the diffractive opticalmodulator of the eighth example of the invention.

FIG. 30 is a side view showing a fundamental configuration of aninfrared sensor according to a ninth example of the invention.

FIG. 31 is a cross-sectional view of a diffractive optical modulatoraccording to a tenth example of the invention.

FIGS. 32A to 32F are cross-sectional views showing the process steps forproducing the diffractive optical modulator of the tenth example of theinvention.

FIG. 33A is a cross-sectional view illustrating the operation of thediffractive optical modulator of the tenth example when no voltage isapplied, while

FIG. 33B is a cross-sectional view illustrating the operation of thediffractive optical modulator of the tenth example when a voltage isapplied.

FIG. 34 is a cross-sectional view of an infrared sensor according to aneleventh example of the invention.

FIG. 35 is a cross-sectional view of an infrared sensor according to atwelfth example of the invention.

FIG. 36 is a cross-sectional view of an infrared sensor according to athirteenth example of the invention.

FIG. 37 schematically shows the principle of the operation of a displaydevice according to the invention.

FIG. 38 schematically shows the arrangement for a display deviceaccording to an example of the invention.

FIG. 39A is a perspective view of a display device according to anotherexample of the invention, while

FIG. 39B is a cross-sectional view taken along the line A-A' in FIG.39A.

FIG. 40A is a plan view of a display device according to still anotherexample of the invention, while

FIG. 40B is a cross-sectional view taken along the line A-A' in FIG.40A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

EXAMPLE 1

Referring to FIGS. 1 to 4, an infrared sensor according to a firstexample of the invention will be described.

The infrared sensor of this example includes a lens provided in anopening of a seal case, and a diffractive optical modulator confined inthe seal case as well as a pyroelectric element. Such an arrangementmakes it possible to produce a micro-infrared sensor. The diffractiveoptical modulator itself can be driven at a low power consumption, andis excellent in the durability and the response speed.

As shown in FIG. 1, the lens for converging infrared light has acorrugated grating structure which corresponds to the phase modulationof the lens. For example, the lens is a diffractive lens 8 having anaperture of 3 mm and a focal length of 6 mm. This lens 8 is composed ofa plurality of concentric grating zones, and the grating period in thezones gradually decreases towards the outer circumference thereof. Thecross section of the grating zones, having a maximum groove depth of 3μm, for example, is in a so-called multi-level shape havingapproximately a saw-tooth shape in four steps, for example. The lens 8converges the incoming light by utilizing the diffraction phenomena.This diffractive lens 8 is formed directly on the reverse side of a Sisubstrate on which a film functioning as infrared wavelength filter isdeposited, and is provided on the opening of the seal case 5. In thisconstruction, a spherical lens made of Si which is required in theconventional example is no longer necessary. Therefore, a smallerinfrared sensor can be constructed more easily and at a lower cost.

This diffractive lens 8 is produced by performing the step of aphotolithography process followed by a reactive ion etching (RIE)process, twice with respect to an entire Si wafer. This method allowsfor producing as many as several hundreds to several thousands of lensessimultaneously per single wafer. In place of the Si, the substrate (orthe wafer) may also be made of Ge, GaAs, InP, GaP, ZnSe or ZnS which istransparent in an infrared region.

The diffractive optical modulator 9 has, for example, an area of 2.5mm×2.5 mm, and a thickness of 0.4 mm and is included in the seal case 5as well as the pyroelectric element 2. The diffractive optical modulator9 is a reflection diffractive optical element which can vary thereflectance (or the zero-order diffraction efficiency) by controllingthe voltage to be applied to the modulator 9. In the coordinate systemsshown in FIGS. 1 and 2, the diffractive optical modulator 9 has adisposition plane (x'y plane) inclined with respect to the dispositionplane (xy plane) of the lens 8 by an angle θ_(t) of 45 degrees, forexample. As a result, as shown in FIG. 1, the optical axis of theconverged infrared light 7 having a wavelength λ of 10 μm is folded bythe diffractive optical modulator 9 by 90 degrees, for example, so thatthe zero-order diffracted light 11 is incident onto the pyroelectricelement 2 disposed on the left of the modulator 9. The zero-orderdiffracted light 11 is directed in the same direction as the directionof the light reflected by an ordinary mirror disposed at the position ofthe modulator 9. This optical system with a folded optical path has anadvantage of reducing the height of the seal case 5.

When a voltage is applied to this diffractive optical modulator 9, thezero-order diffracted light 11 disappears and first-order diffractedlight 10a and minus first-order diffracted light 10b are generated asindicated by the broken lines in FIG. 2 so that the ratio of each of thetwo lights 10a and 10b to the entire light is 0.41. These two diffractedlights 10a and 10b are then converged outside of the pyroelectricelement 2. Consequently, the infrared light is not incident on thepyroelectric element 2. Therefore, by turning on/off the voltage appliedto the diffractive optical modulator 9, the amount of the infrared lightincident on the pyroelectric element 2 can be varied, and therefore thediffractive optical modulator 9 can operate in the same way as aconventional chopper.

As shown in FIG. 2 and FIGS. 3A and 3B, this diffractive opticalmodulator 9 includes a Si substrate 14 and a grating 12 consisting of aplurality of beams 12₁, 12₂ . . . 12_(N) which can move vertically. Thegrating 12 is formed by an SiN layer 16 or the like, and has a length of2 mm, a thickness L of 3.5 μm, and a period Λ of 100 μm.

An SiO₂ layer 13 is provided as a spacer layer between the grating 12and the substrate 14. Both ends of each beam of the grating 12 aresupported by this spacer layer. The distance S between the grating 12and the substrate 14 is 3.5 μm, for example. This distance can beadjusted by varying the thickness of the spacer layer.

Reflection films 15a and 15b made of Au are provided on the surface ofthe grating 12 and on the surface of the substrate 14. The reflectionfilm 15a is insulated with the substrate 14 by the spacer layer (SiO₂layer) 13.

In this example, an entire plate-type Si substrate 14 functions as afirst electrode. The reflection film 15a functions as a secondelectrode. By applying a voltage between the substrate 14 and thereflection film 15a, an electrostatic force is generated therebetween,and the supported portions of the grating 12 in the vicinity of bothends thereof are deflected, so that the grating 12 comes into contactwith the substrate 14 as shown in FIG. 3B. The length of the grating 12is set to be larger than the aperture of the beam (indicated by thebroken lines in FIG. 2), e.g., 1.8 mm, when the converged infrared light7 is reflected by the diffractive optical modulator 9, so that the lightis well modulated. However, if the length is set to be too large, thenunnecessary infrared light incident on the regions other than thedetection region is possibly diffracted in the peripheral region of thegrating 12 which is not to be used, and then incident onto thepyroelectric element 2. Thus, in this example, the length of the grating12 is set to be larger than the aperture of the reflected beam by about5 to 20%.

The present inventors have found that the optimal thickness of thegrating 12 and the optimal height of the space (movable distance)depends upon the inclination angle θ_(t) of the diffractive opticalmodulator 9; that the thickness L of the SiN layer 16 is λ/(4 cosθ_(t)), e.g., 3.5 μm, where λ is a wavelength of the incoming light; andthat the height S of the space between the grating 12 and the substrate14 is also λ/(4 cos θ_(t)), e.g., 3.5 μm. When no voltage is applied,the phase shift between the light reflected in the Z' direction by thereflection layer 15a on the surface of the grating 12 and the lightreflected by the reflection layer 15b on the surface of the substrate14, becomes 2π. Since the phases are exactly matched, the diffractiveoptical modulator 9 functions merely as a mirror, and the diffractionefficiency of the zero-order diffracted light becomes approximately 100%except for the reflection loss, so that only the zero-order diffracted(or reflected) light 11 is generated. On the other hand, when a voltageis applied, the phase shift between the light reflected by thereflection layer 15a on the surface of the grating 12 and the lightreflected by the reflection layer 15b on the surface of the substrate14, respectively, becomes n. In this case, since the phases are entirelyopposite to each other, the zero-order diffraction efficiency becomes0%, and the ±first-order diffracted lights 10a and 10b are generated atthe diffraction efficiency of 41%, respectively. However, even if theheight S and the thickness L are varied more or less, the diffractiveoptical modulator 9 can still function as a chopper. In the prior art asdisclosed by O. Solgaard, F. S. A. Sandejas and D. M. Bloom in"Deformable Grating Optical Modulator," Optics Letters, Vol. 17, No. 9,May 1, 1992, collimated visible light (having a wavelength λ of 0.6328μm, for example) is incident vertically onto the device (or thediffractive optical modulator), so that it is difficult to separate theincoming light from the reflected zero-order diffracted light. Inaddition, the grating period Λ may be 2, 3 or 4 μm, but the gratingwidth becomes a half of the grating period, i.e., 1, 1.5 or 2 μm. Sincethe grating width is too small, it is difficult to fabricate thegrating. Furthermore, since the grating thickness or the height of thespace is very small, e.g., about 0.2 μm, the error is likely to occur inthe thickness of the deposited film during the production process.Accordingly, it is difficult to produce a diffractive optical modulatorwith excellent properties in a high production yield.

On the other hand, the infrared sensor according to the presentinvention uses an infrared ray having a central wavelength of 10 μm asthe incoming light, so that the width of the grating to be producedbecomes very large, e.g., about 50 μm. Accordingly, the patterningprocess can be easily performed for the grating. In addition, both theoptimal grating thickness and the optimal height of the space becomevery large, e.g., about 3.5 μm, the thickness of the thin film to bedeposited can be controlled very satisfactorily. Moreover, the grains orthe fine unevenness generally caused in a thin film give adverse effectswhen visible light is incident, e.g., the scattering of the light.However, the infrared sensor of the invention is not affected in any ofthe above ways.

In this example, the direction of the grating vector (or y direction),which is vertical to the longitudinal direction (or x' direction) of thediffractive optical modulator 9 and is on the surface where the gratingis provided, is vertical to the optical axis (or -z direction) of theinfrared light 7 converged by the lens 8, and the disposition plane (x'yplane) of the diffractive optical modulator 9 is inclined with respectto the disposition plane (xy plane) of the lens 8. The present inventorshave found that by using such a configuration, the incoming convergedinfrared light 7 can be satisfactorily separated from the outputzero-order diffracted light 11, and that the zero-order diffractionefficiency when no voltage is applied is approximately 100%.

In the infrared sensor of the invention, not the collimated light butthe converged light is incident onto the diffractive optical modulator9. The present inventors have found that the zero-order diffractionefficiency becomes approximately 100% in the central beams of thegrating 12 in the y direction, e.g., when the numbers of the beams areten, beams 12₄, 12₅ and 12₆, when no voltage is applied, but that thezero-order diffraction efficiency gradually decreases in the peripheralbeams of the grating 12 in the y' direction, e.g., beams 12₁, 12₂, 12₉and 12₁₀, because the incident angle of the light is inclined. Becauseof the same reasons, when a voltage is applied, the zero-orderdiffraction efficiency increases from 0% in the peripheral beams, andtherefore the width of the varying light amount reduces as a whole.However, as discovered by the present inventors, if the period Λ of thegrating 12 is 7 times or more of the wavelength of the infrared light,i.e., Λ/λ≧7, then the variation of the diffraction efficiency is smalleven in the light obliquely incoming, and the incoming converged lightcauses no problem in the infrared sensor of the invention. In addition,in the case where Λ/λ≧7, the line width of the grating to be produced isas large as 35 μm or more, the grating can be produced easily.

In this example, the diffractive optical modulator is produced by usinga Si substrate. Alternatively, a substrate obtained by forming aconductive layer on an insulating substrate such as a glass substratemay also be used. Any substrate can be used so long as the substrate isa plate shaped element for supporting the grating and has a portion forfunctioning as a first electrode for generating an electrostatic forcebetween the grating and the substrate. More preferably, a semiconductorsubstrate is used, because fine patterning processes such as aphotolithography technique and an etching technique which have beendeveloped in the field of producing a semiconductor device are usedduring the process for producing the grating. This is true of all thefollowing examples.

EXAMPLE 2

Referring to FIG. 5 and FIG. 6, an infrared sensor according to a secondexample of the invention will be described.

The infrared sensor of this second example is different from theinfrared sensor of the first example only in the configuration of thediffractive optical modulator. Therefore, the configuration of thediffractive optical modulator will be described below.

In the diffractive optical modulator 9' to be used in this example, asshown in FIG. 5, the thickness L of the grating 12' is varied in therespective positions. That is to say, the thickness L becomes smalleston the optical axis of the incoming infrared light 7 (the thickness L₄is λ/(4 cos θ_(t)), for example), and the thickness gradually increasestowards the periphery of the grating 12' (L₁ >L₂ >L₃ >L₄ <L₅ <L₆ <L₇).The same relationship is applied to the height S of the space betweenthe grating 12' and the substrate 14. More specifically, the verticallymovable distance of the grating 12' is also smallest on the optical axisof the incoming infrared light 7 (the height S₄ is λ/(4 cos θ_(t)), forexample), and the thickness gradually increases towards the periphery ofthe grating 12' (S₁ >S₂ >S₃ >S₄ <S₅ <S₆ <S₇). In FIG. 5, a diffractiveoptical modulator has seven beams in the grating 12'. However, thenumber of the beams included in the grating 12' is not limited thereto.

The present inventors have found when the expression L=λ/(4 cos θ·cosθ_(t)) is satisfied, where L is the thickness of the grating 12' and θis the incident angle of the infrared light 7 being incident on theportion of the grating 12', and when the expression S=λ/(4 cos θ·cosθ_(t)) is satisfied, where S is the height of the space, the decrease inthe zero-order diffraction efficiency when no voltage is applied and theincrease in the zero-order diffraction efficiency when a voltage isapplied can be prevented and the modulation efficiency can be large inthe peripheral portions onto which the oblique light is incoming.Accordingly, even if Λ/λ<7 and the period of the grating is small, themodulation efficiency of the zero-order diffracted light 11 can belarge. If either the thickness L or the height S has an optimumdistribution, the modulation efficiency is improved.

The diffractive optical modulator shown in FIG. 5 is produced in thefollowing manner.

First, an SiO₂ thin film 13 is deposited on a Si substrate 14 as a firstthin film for defining the distance between the grating 12 and the Sisubstrate 14. Next, an SiN thin film 16 is deposited as a second thinfilm to be a grating 12. When the first thin film is being deposited, anelongated shield 18 extending in the x' direction is moved in the ydirection between the source 17 for depositing the thin film and thesubstrate 14 as shown in FIG. 6. By controlling the speed of the shield18 in the y direction, the deposition amount of the thin film is alsocontrolled. As a result, the thickness of the SiO₂ thin film 13 can becontrolled. If the speed of the moving shield. 18 is set to be lowaround the center while high in the periphery, the SiO₂ thin film 13whose thickness is distributed as shown in FIG. 6 can be obtained. If asimilar process is performed during the deposition of the second thinfilm, the thickness of the SiN thin film 16 can be distributed in thesame way as the thickness of the SiO₂ thin film 13 as shown in FIG. 6.

Thereafter, by performing the photolithography process and the etchingprocess, the grating as shown in FIG. 5 can be formed.

On the other hand, by using a shield 18 having a plurality of elongatedportions extending in the x' direction, an array of diffractive opticalmodulators 9' can be produced easily.

EXAMPLE 3

Referring to FIGS. 7A and 7B, an infrared sensor according to a thirdexample of the invention will be described. The infrared sensor of thisthird example is different from the infrared sensor of the first exampleonly in the configuration of the diffractive optical modulator.Therefore, the configuration of the diffractive optical modulator willbe described below.

The diffractive optical modulator of this example is designed so as tomodulate the infrared light having an incident angle θ_(t) of 45 degreesand a wavelength λ of 10.6 μm. Needless to say, a diffractive opticalmodulator having a different configuration can also be used.

In FIGS. 7A and 7B, the diffractive optical modulator includes: asubstrate 21, e.g., a silicon substrate in this example; an insulatinglayer 22; a spacer layer 23 made of a positive type photoresist having athickness λ/(4 cos θ_(t)) of 3.75 μm, in this example; a conductive thinfilm 24 obtained by depositing aluminum or the like so as to be 3.75 μmthick; and a reflection film 25 obtained by depositing Au.

Hereinafter, referring to FIGS. 8A to 8G, a method for producing adiffractive optical modulator of this example will be described.

In these figures, the reference numeral 26 denotes a mask for patterningthe conductive thin film 24; 27a to 27d denote beams; 28a to 28e denoteopenings; 29a to 29d denote upper reflection films; and 30a to 30edenote lower reflection films. The grating consists of the upperreflection films 29a to 29d and the beams 27a to 27d.

First, as shown in FIG. 8A, a silicon substrate 21 is thermally oxidizedat 1050° C. for an hour, thereby forming an insulating layer 22 formedby the thermally oxidized film having a thickness of 0.1 μm on thesilicon substrate 21.

Next, as shown in FIG. 8B, a photoresist is applied onto the insulatinglayer 22 by a spin-coating method, thereby forming a spacer layer 23.The photoresist is baked at 160° C. for 20 minutes after the photoresisthas been applied. In this example, by adjusting the rotation number ofthe spinner and the viscosity and the temperature of the photoresist,the thickness of the spacer layer 23 after the baking is set to be 3.75μm.

Then, as shown in FIG. 8C, an aluminum film having a thickness of 3.75μm is deposited on the spacer layer 23 by a vapor deposition method or asputtering method so as to form a conductive thin film 24.

Next, as shown in FIG. 8D, a photoresist is applied onto the conductivethin film 24, and then exposed and developed so as to form a mask 26.

Subsequently, as shown in FIG. 8E, aluminum forming the conductive thinfilm 24 is removed by a wet etching (hereinafter referred to as a W/E)using an etching solution composed of phosphoric acid, acetic acid andnitric acid, thereby forming the openings 28a to 28e in the conductivethin film 24 and a plurality of beams 27a to 27d made of a conductivematerial. In this example, in order to control the incoming light havinga diameter of 1.8 mm, the length (measured along the direction verticalto the paper sheet) of the beams 27a to 27d is set to be 2 mm.

Then, as shown in FIG. 8F, the mask 26 and the spacer layer 23 areremoved by an isotropic dry etching (hereinafter, referred to as a D/E).The portions of the spacer layer 23 which are located under the beams27a to 27d are also isotropicly etched, so that the spacer layer 23remains only in the peripheral portions of the substrate 21 as shown inFIG. 8F, and supports both ends of the beams 27a to 27d.

Finally, as shown in FIG. 8G, Au is deposited on the upper surface ofthe substrate 21, thereby forming a reflection film 25 having athickness of 0.15 μm. The reflection film 25 includes the upperreflection films 29a to 29d formed on the beams 27a to 27d and the lowerreflection films 30a to 30e which are formed on the upper surface of theinsulating film 22 so as to be exposed through the openings 28a to 28e.

In the case of forming a grating in which the length of a beam isconsiderably longer than the width of the beam, if the spacer layer 23is removed by the W/E, then the surface tension of the liquid causedduring the rinsing and the drying processes unexpectedly sticks thebeams onto the substrate side. However, according to the productionmethod of this example, since the spacer layer 23 is selectively removedby the DE, the problem of the sticking can be totally eliminated, andthe production yield can be considerably improved.

The length of a beam is varied how far the spacer layer 23 is etched inthe lateral direction from the openings 28a to 28e (or in the verticaldirection in FIG. 7A). In the diffractive optical modulator of theinvention, unless the length of each of the beams 27a to 27d is set tobe constant, the restoration rate of the respective beams are variedwhen the modulator is driven and the diffraction of the light becomesununiform for some time. If the spacer layer 23 is removed by the W/E,then the amounts of the etchant flowing under the respective beams 27ato 27d and the time required for removing the etchant at the time ofrinsing are slightly different among the respective beams 27a to 27d, sothat the resulting lengths of the beams 27a to 27d are varied in thediffractive optical modulator. However, in the case of removing thespacer layer 23 by the D/E, the lengths of the beams 27a to 27d becomealmost the same.

Next, referring to FIGS. 9A and 9B, the operation of the diffractiveoptical modulator of this example will be described.

In FIGS. 9A and 9B, the reference numeral 31 denotes incoming light; 32denotes the zero-order diffracted light, 33a to 33d denote the airlayers formed by the beams 27a to 27d suspended in the air by theisotropic etching of the spacer layer 23; 34 denotes an upper electrodeconsisting of the conductive thin film 24 and the reflection film 25;and 35a and 35b denote ± first-order diffracted lights. FIG. 9A showsthe state where no voltage is applied between the upper electrode 34 andthe substrate 21. The difference in the height between the upperreflection films 29a to 29d and the lower reflection films 30a to 30e is7.5 μm, for example.

In the state where a positive voltage is applied between the conductivethin film 24 and the substrate 21, the beams 27a to 27d and the lowerelectrode, i.e., the substrate 21, form a capacitor which consists ofthe air layers 33a to 33d and the insulating layer 22. As a result, thebeams 27a to 27d serving as the upper electrode are charged withpositive charges, while the substrate 21 is charged with negativecharges. Since an electrostatic attracting force is caused between thesecharges, the beams 27a to 27d are pulled toward the insulating layer 22until the beams 27a to 27d come into contact with the insulating layer22, as shown in FIG. 9B. In this stage, the difference in the heightbetween the surfaces of the upper reflection films 29a to 29d and thoseof the lower reflection films 30a to 30e is 3.75 μm, for example.

As is apparent from the foregoing description, since the diffractiveoptical modulator of this third example operates in a similar manner tothe diffractive optical modulator of the first example, the diffractiveoptical modulator of the third example also allows for modulatingincoming light having a wavelength of 10.6 μm, for example, by turningon/off the voltage to be applied between the conductive thin film 24 andthe substrate 21.

In the diffractive optical modulator of the third example, the distancebetween the conductive thin film 24 functioning as the upper electrodeand the substrate 21 functioning as the lower electrode is smaller thanthat in the diffractive optical modulator of the first example, so thata stronger electrostatic force is caused between them. Therefore, themodulator of this example can be advantageously driven at a lowervoltage.

As described above, according to the method of this example, the spacerlayer is formed by an organic film and is removed by the D/E, so thatthe beams are not sticked onto the substrate during the rinsing anddrying processes, and the spacer layer can be removed uniformly withsatisfactory reproducibility. Consequently, a plurality of beams havingthe same length are formed, and the variation of the operation can beeliminated. In addition, as compared with the diffractive opticalmodulator of the first example, the gap between the upper and the lowerelectrodes becomes smaller in the diffractive optical modulator of thethird example, so that the modulator of the third example can be drivenat a lower voltage.

EXAMPLE 4

Referring to FIG. 10, an infrared sensor according to a fourth exampleof the invention will be described. The infrared sensor of this fourthexample is different from the infrared sensor of the first example onlyin the configuration of the diffractive optical modulator. Therefore,the configuration of the diffractive optical modulator will be describedbelow.

In FIG. 10, the diffractive optical modulator includes: a substrate 36,e.g., a silicon substrate in this example; an insulating layer 37; aspacer layer 38 made of a photoresist having a thickness of 3.75 μm, inthis example; a conductive thin film 39, functioning as both the upperelectrode and the upper reflection film, obtained by depositing aluminumor the like so as to be 3.75 μm thick, in this example; and lowerreflection films 40a to 40e made of the same material as that of theconductive thin film 39 or the material having substantially the samereflectance as that of the conductive thin film 39, e.g., aluminum inthis example.

Hereinafter, referring to FIGS. 11A to 11I, a method for producing adiffractive optical modulator of this example will be described. InFIGS. 11A to 11I, the reference numeral 41 denotes a reflection film and42a to 42e denote masks.

First, as shown in FIG. 11A, a silicon substrate 36 is thermallyoxidized at 1050° C. for an hour, thereby forming an insulating layer 37constituted by the thermally oxidized film having a thickness of 0.1 μmon the silicon substrate 36.

Next, as shown in FIG. 11B, aluminum is deposited on the insulatinglayer 37 by a vapor deposition method so as to be 0.15 μm thick, therebyforming a reflection film 41. A photoresist is applied onto thereflection film 41 by a spin-coating method, and then exposed anddeveloped, thereby forming the masks 42a to 42e.

Subsequently, as shown in FIG. 11C, aluminum forming the reflection film41 is removed by a W/E using an etching solution composed of phosphoricacid, acetic acid and nitric acid, for example.

Then, as shown in FIG. 11D, the masks 42a to 42e are removed, therebyforming the lower reflection films 40a to 40e.

Next, as shown in FIG. 11E, a photoresist is applied by a spin-coatingmethod, and then baked, thereby forming a spacer layer 38. In thisexample, by adjusting the rotation number of the spinner and theviscosity and the temperature of the photoresist, the thickness of thespacer layer 38 after the baking is set to be 3.75 μm, for example, andthe surface of the spacer layer 38 is made flat.

Then, as shown in FIG. 11F, an aluminum film having a thickness of 3.9μm, for example, is deposited on the spacer layer 38 by a vapordeposition method, thereby forming the conductive thin film 39. Sincethe thickness of the lower reflection films 40a to 40e is 0.15 μm, forexample, the thickness of the conductive thin film 39 is also set to belarger than 3.75 μm, for example, by 0.15 μm, i.e., 3.9 μm.

Thereafter, as shown in FIG. 11G, a photoresist is applied onto theconductive thin film 39, and then exposed and developed, thereby formingthe mask 43.

Subsequently, as shown in FIG. 11H, aluminum forming the conductive thinfilm 39 is removed by a W/E using an etching solution composed ofphosphoric acid, acetic acid and nitric acid, for example, therebyforming the beams 44a to 44d and the openings 45a to 45e.

Finally, as shown in FIG. 11I, the mask 43 is removed by the D/E, and atthe same time, the portions of the spacer layer 38 under the beams 44ato 44d are also isotropicly etched.

By performing the above process steps, the diffractive optical modulatorhaving a configuration shown in FIG. 10 is completed.

Hereinafter, the operation of the diffractive optical modulator havingthe above-described configuration will be described with reference toFIGS. 12A and 12B. In FIGS. 12A and 12B, the same components as thoseshown in FIG. 10 are denoted by the same reference numerals and thedescription thereof will be omitted herein. In FIGS. 12A and 12B, thereference numeral 46 denotes incoming light; 47 denotes the zero-orderdiffracted light; 48a to 48d denote the air layers formed by the beams44a to 44d suspended in the air by the isotropic etching of the spacerlayer 38; and 49a and 49b denote ± first-order diffracted lights. FIG.12A shows the state where no voltage is applied between the conductivethin film 39 and the substrate 36. The difference in the height betweenthe surfaces of the beams 44a to 44d and those of the lower reflectionfilms 40a to 40e is 7.5 μm, for example.

In the state where a positive voltage is applied between the conductivethin film 39 and the substrate 36, the beams 44a to 44d and the lowerelectrode, i.e., the substrate 36, form a capacitor so as to interposethe air layers 48a to 48d and the insulating layer 37. As a result, thebeams 44a to 44d serving as the upper electrode are charged withpositive charges while the substrate 36 is charged with negativecharges. Since an electrostatic attracting force is caused between thesecharges, the beams 44a to 44d are pulled toward the insulating layer 37until the beams 44a to 44d come into contact with the insulating layer37, as shown in FIG. 12B. In this stage, the difference in the heightbetween the surfaces of the beams 44a to 44d and those of the lowerreflection films 40a to 40e is 3.75 μm, for example.

As is apparent from the foregoing description, since the diffractiveoptical modulator of this fourth example operates in a similar manner tothe diffractive optical modulator of the first example, the diffractiveoptical modulator of the fourth example also allows for modulatingincoming light having a wavelength of 10.6 μm, for example, by turningon/off the voltage to be applied between the conductive thin film 39 andthe substrate 36.

In this example, since the lower reflection films are formed beforehand,the beams can function as the upper reflection film and an ultimatereflection film need not be deposited. Accordingly, especially in thecase where a thick reflection film is required, an inadequate operationcaused by the contact between the lower reflection films and thereflection film attached to the sides of the beams can be completelyeliminated.

The conductive thin film is made of aluminum in this example. Needlessto say, the conductive thin film may be made of other materials. In thisexample, the aluminum film is deposited by a vapor deposition method.However, the aluminum film may be deposited by a sputtering method or aplating method. The spacer layer is made of a photoresist.Alternatively, the spacer layer may be made of an organic material suchas polyimide.

EXAMPLE 5

Referring to FIG. 13, an infrared sensor according to a fifth example ofthe invention will be described.

In FIG. 13, the diffractive optical modulator of the fifth exampleincludes: a substrate 50, e.g., a silicon substrate in this example; aninsulating layer 51 which consists of a thermally oxidized film having athickness of 0.1 μm, for example, obtained by thermally oxidizing thesubstrate 50, and a nitride film having a thickness of 0.5 μm, forexample, obtained by a low-pressure chemical vapor deposition (LPCVD)method; a spacer layer 52 having a thickness of 3.75 μm, for example,obtained by depositing a silicon oxide film by the LPCVD method; adielectric layer 53 having a thickness of 3.75 μm, for example, to beobtained by depositing a nitride film by the LPCVD method, thedielectric layer 53 being patterned so as to form the beams 54a to 54dsupported at both ends thereof and having a reduced residual tensilestress, e.g., about 200 MPa, by using a silicon-rich composition for thenitride film; the beams 54a to 54d, both ends of which are supported bythe spacer layer 52, suspended in the air; openings 55a to 55e; and areflection film 56 obtained by depositing an Au thin film having athickness of 0.15 μm, for example, by a vapor deposition method. Asshown in FIG. 13, the reflection film 56 forms not only the upperreflection films 57a to 57d on the surfaces of the beams 54a to 54d, butalso the lower reflection films 58a to 58e on the surfaces of theinsulating layer 51 through the openings 55a to 55e. A grating is formedby these upper reflection films 57a to 57d and the beams 54a to 54d.

Hereinafter, referring to FIGS. 14A to 14G, a method for producing adiffractive optical modulator of this example will be described.

First, as shown in FIG. 14A, a silicon substrate 50 is thermallyoxidized at 1050° C. for an hour, for example, thereby forming athermally oxidized film having a thickness of 0.1 μm. Thereafter, asilicon nitride film having a thickness of 0.5 μm, for example, isdeposited thereon by the LPCVD, thereby forming an insulating layer 51.

Next, as shown in FIG. 14B, a spacer layer 52 formed by a silicon oxidefilm is deposited on the insulating layer 51 by the LPCVD.

Then, as shown in FIG. 14C, a silicon-rich silicon nitride film isdeposited on the spacer layer 52 by the LPCVD, thereby forming thedielectric layer 53.

Subsequently, as shown in FIG. 14D, a photoresist 59 is applied onto thedielectric layer 53 by a spin-coating method, and then exposed anddeveloped, thereby forming a mask in a predetermined shape.

Next, as shown in FIG. 14E, the dielectric layer 53 is patterned by theD/E, thereby forming the beams 54a to 54d.

Then, as shown in FIG. 14F, the photoresist 59 is removed, and thespacer layer 52 is removed by the W/E using buffered hydrofluoric acidso as to remove the spacer layer 52 under the beams 54a to 54d and formthe beams whose both ends are supported.

Finally, as shown in FIG. 14G, a reflection film 56 made of Au having athickness of 0.15 μm, for example, is deposited by a sputtering method,thereby forming the upper reflection films 57a to 57d and the lowerreflection films 58a to 58e. By performing the above process steps, thediffractive optical modulator having the configuration shown in FIG. 13is completed.

Since the diffractive optical modulator having the above configurationoperates in a similar manner to the diffractive optical modulators ofthe third or the fourth example, the diffractive optical modulator ofthe fifth example also allows for modulating incoming light having awavelength of 10.6 μm, for example, by turning on/off the voltage to beapplied between the reflection film 56 functioning as the upperelectrode and the substrate 50 functioning as the lower electrode, asshown in FIG. 13.

In the diffractive optical modulator of this example, since a nitridefilm is used as the material for the beams, a residual tensile stress(of, for example, 200 MPa) is caused. As a result, the driving voltagebecomes considerably higher as compared with that of the diffractiveoptical modulator of the third or the fourth example. However, a beamhaving a length much larger than the thickness thereof can beadvantageously formed. Therefore, incoming light having a large diametercan be modulated. In the diffractive optical modulator of the firstexample having no insulating layer 51, when a reflection film is beingdeposited, the reflection film unexpectedly reaches the sides of thebeams, so that the upper reflection films become electrically conductivewith the substrate when the modulator is driven. As a result, anelectric current is generated therebetween, thereby preventing themodulator from operating adequately, and the production yield isdisadvantageously reduced. However, in the diffractive optical modulatorof this fifth example, an insulating layer 51 is provided between thelower reflection films 58a to 58e and the substrate 50. Accordingly,when the reflection film 56 is being deposited, even if the reflectionfilm 56 reaches the sides of the beams 54a to 54d, and the upperreflection films 57a to 57d become electrically conductive with thelower reflection films 58a to 58e, an electrical conduction between theupper reflection films 57a to 57d and the substrate 50 can be totallyeliminated, because the lower reflection films 58a to 58e areelectrically insulated with the substrate 50. Consequently, it ispossible to provide a diffractive optical modulator operatingappropriately, and improve the production yield.

As described above, in the diffractive optical modulator of the firstexample, when the reflection film is being deposited, the reflectionfilm reaches the sides of the substrate, and when the modulator isdriven, the upper reflection films come into contact with the lowerreflection films, so that a short-circuit is generated, and themodulator tends to operate inadequately. However, in the diffractiveoptical modulator of this fifth example, since the insulating layer isprovided between the lower reflection films and the substrate, themodulator can operate stably.

Next, referring to FIGS. 9A and 9B and FIGS. 15A to 15C, a method forapplying a voltage to the diffractive optical modulator of the inventionwill be described.

FIG. 15A shows the waveform of the voltage to be applied to thediffractive optical modulator by a voltage application device. Thedriving voltage having such a waveform corresponds to the case where themodulation of the light is generated at a period of 20 msec(corresponding to a frequency of 50 Hz), for example, as described inthe third or the fourth example.

The driving voltage shown in FIG. 15A has a waveform in which thepolarity in the region AB is opposite to the polarity in the region CD,but the levels in these two regions AB and CD are the same. The region Ashown in FIG. 15A corresponds to the state shown in FIG. 9B where avoltage of ±15 V, for example, is applied between the upper electrode 34and the substrate 21. In this case, as described in the third, fourth orfifth example, the upper electrode 34 is charged with positive charges,while the substrate 21 is charged with negative charges. As a result,the beams 27a to 27d are deflected, and sticked onto the insulatinglayer 22, as shown in FIG. 9B. The next region B shown in FIG. 15Acorresponds to the state where no voltage is applied and almost all thecharges are removed. However, the present inventors have found that someof the charges remain as the residual charges. Since the restorationforce of the beams 27a to 27d is stronger than the sticking force of theresidual charges, the beams 27a to 27d are restored to the initialpositions thereof, and suspended in the air as shown in FIG. 9A. Inorder to operate the beams 27a to 27d by the application of a voltagehaving the same polarity next time, a voltage higher than the previousvoltage is required to be applied because of the effects of the residualcharges. Otherwise, the beams 27a to 27d are not sticked onto theinsulating layer 22. Therefore, in the next region C, by applying avoltage having an opposite polarity to that of the voltage applied inthe region A, e.g., a voltage of -15 V, the residual charges remainingin the respective electrodes are forcibly removed, and at the same time,the charges having an opposite polarity are charged. As a result, thebeams 27a to 27d are sticked onto the insulating layer 22 again. Theregion D as well as the region B corresponds to the state where novoltage is applied, and some of the charges also remain as the residualcharges in the region D.

As described above, in the diffractive optical modulator of thisexample, the residual charges caused when the driving voltage is appliedcan be removed by alternately applying voltages having oppositepolarities. In a conventional diffractive optical modulator, since theresidual charges increase as a modulator is driven more times, thenecessary driving voltage is required to be higher to remove theincreasing residual charges. However, in this example, such a problemcan be solved.

The same operation can be obtained if the application of a voltage isbegun by a voltage having an opposite polarity to that of the voltageshown in FIG. 15A.

FIG. 15B shows a waveform of another driving voltage.

The region A shown in FIG. 15B corresponds to the state shown in FIG. 9Bwhere a voltage of +50 V, for example, is applied between the upperelectrode 34 and the substrate 21. In this case, the upper electrode 34is charged with positive charges, while the substrate 21 is charged withnegative charges. As a result, the beams 27a to 27d are deflected, andsticked onto the insulating layer 22, as shown in FIG. 9B. The nextregions B and C shown in FIG. 15B correspond to the state shown in FIG.9A. In the region B, in order to promote the removal of the residualcharges, a voltage having an opposite polarity, e.g., a voltage of -50V, is applied during an initial short period.

FIG. 15C shows a waveform of still another driving voltage. The voltagewaveform shown in FIG. 15C is different from the voltage waveform shownin FIG. 15B in that a voltage not having an opposite polarity but lowerthan the voltage in the region A is applied during an initial shortperiod in the region B. By applying a voltage having such a waveform, itis possible to promote the removal of the charges and to restore thebeams at a high speed.

EXAMPLE 6

Hereinafter, referring to FIG. 16, an infrared sensor according to asixth example of the invention will be described. The infrared sensor ofthis example is designed so as to detect infrared light having awavelength of 10 μm, for example.

The infrared sensor of this example includes: a lens provided in theopening of a seal case; and a diffractive optical modulator included inthe seal case as well as a pyroelectric element. The size of theinfrared sensor of this example is reduced as compared with aconventional infrared sensor using a chopper. In addition, thediffractive optical modulator itself can be driven at a low powerconsumption, and is excellent in the durability and the response speed.

First, the configuration of the infrared sensor of this example will bedescribed.

In FIG. 16, the reference numeral 61 denotes a seal case. A part of theseal case 61 is omitted herein for the purpose of the illustration ofthe inside thereof. A lens 62 is a diffractive lens having an apertureof 3 mm and a focal length of 6 mm, for example, which is formed on asubstrate made of a material having transparency in the infrared region,e.g., silicon. A diffractive optical modulator 63 modulates infraredlight having a wavelength of 10 μm, for example. A spacer 64 has aninclination angle of θ_(t). Though not shown in FIG. 16, on the reverseside of a supporting plate 65, electronic parts such as a pyroelectricelement and a signal amplifier are disposed. Electrode pins are denotedby 66a to 66d, however, the electrode pin 66d is not shown in FIG. 16,because the electrode pin 66d is disposed in an invisible position inFIG. 16. The electrode pins 66a to 66d are used for grounding, anapplication of a voltage to the diffractive optical modulator 63, and asupply of power or an output of a signal to a pyroelectric element andother electronic parts not shown in FIG. 16.

FIG. 17 is a perspective view showing the supporting plate 65 seen fromthe bottom side in FIG. 16. In FIG. 17, the same components as those inFIG. 16 are denoted by the same reference numerals, and the descriptionthereof will be omitted herein. In FIG. 17, a pyroelectric element isdenoted by 67, and a signal amplifier for amplifying the signal outputfrom the pyroelectric element 67 is denoted by 68.

FIG. 18 is a side view showing a fundamental configuration of theinfrared sensor of this example. The pyroelectric element 67 is disposedin the vicinity of the end of the supporting plate 65, whereby theinclination angle θ_(t) is set to be as small as possible. As will bedescribed in detail later, the smaller the inclination angle θ_(t) is,the lower the driving voltage becomes. In this case, if the focal lengthof the lens 62 is 10 mm, for example, then the inclination angle θ_(t)becomes 17 degrees. As a result, the increase in the driving voltage issuppressed to about 10% as compared with the case where the inclinationangle θ_(t) is zero. In addition, the infrared sensor of this examplehas a simple configuration. That is to say, the pyroelectric element 67and the signal amplifier 68 are disposed on the supporting plate 65, andthe supporting plate 65 is supported by at least one of the electrodepins 66a to 66d. Accordingly, it is not particularly necessary toprovide complicated lines and the sealing can be performed easily.

FIG. 19A is a plan view showing a diffractive optical modulator of thisexample, while FIG. 19B is a cross-sectional view taken along the lineA-A' in FIG. 19A. In FIGS. 19A and 19B, the diffractive opticalmodulator includes: a substrate 69, e.g., a silicon substrate; aninsulating layer 70 formed by an oxide film having a thickness of 0.1 μmobtained by thermally oxidizing the silicon substrate 69; a spacer layer71 obtained by applying polyimide by a spin-coating method and bakingthe applied polyimide; and beams 72. The present inventors have foundthat both the optimal thickness of the spacer layer and that of thebeams for maximizing the degree of the modulation are λ/(4 cos θ_(t)),where λ is the wavelength of the light to be detected. As can beunderstood from this expression, the larger the inclination angle θ_(t)becomes, the larger the optimal thickness of the spacer layer and thebeams of the diffractive optical modulator becomes. For example, if λ is10 μm and θ_(t) is 25 degrees, then the thickness of the respectivelayers is 2.8 μm. And if θ_(t) is 45 degrees, then the thickness is 3.5μm. The diffractive optical modulator further includes: a reflectionfilm 73 obtained by depositing Au or the like by a vapor depositionmethod so as to be 0.1 μm thick; a non oxidized conductive thin film 74made of a material which is not likely to be oxidized by oxygen in theatmosphere or the oxygen plasma, to be obtained by depositing Au or Pt;and an elastic body 75 obtained by depositing Al for example. As shownin FIG. 18, the beams 72 consist of the non-oxidized conductive thinfilm 74 and the elastic body 75.

Hereinafter, referring to FIGS. 20A to 20F, a method for producing adiffractive optical modulator of this example will be described.

First, as shown in FIG. 20A, a silicon substrate 69 is thermallyoxidized at 1050° C. for an hour, for example, thereby forming aninsulating layer 70 made of the thermally oxidized film having athickness of 0.1 μm on the silicon substrate 69. Thereafter, polyimideis applied onto the insulating layer 70 by a spin-coating method,thereby forming a spacer layer 71. The applied polyimide is baked at200° C. for 20 minutes. In this example, by adjusting the rotationnumber of the spinner and the viscosity and the temperature ofpolyimide, the thickness of the spacer layer 71 after the baking is setto be 2.8 μm, for example.

Next, as shown in FIG. 20B, Au (thickness: 0.1 μm) is deposited on thespacer layer 71 by a vapor deposition method so as to form anon-oxidized conductive thin film 74, and then an Al film having athickness of 2.7 μm is deposited thereon by a vapor deposition method soas to form an elastic body 75.

Then, as shown in FIG. 20C, a positive type photoresist is applied ontothe elastic body 75, and then exposed and developed so as to form a mask76. Thereafter, the elastic body 75 is removed by the W/E using anetching solution composed of phosphoric acid, acetic acid and nitricacid.

Subsequently, as shown in FIG. 20D, the non-oxidized conductive thinfilm 74 is etched by the D/E. Thereafter, as shown in FIG. 20E, the mask76 is removed by the D/E, and at the same time, the spacer layer 71 isisotropicly etched so that the portions under the beams 72 are alsoetched.

Finally, as shown in FIG. 20F, Au (thickness: 0.1 μm) is deposited so asto form the reflection film 73. As a result, the diffractive opticalmodulator having a configuration shown in FIG. 19 is completed.

The operation of the diffractive optical modulator having the aboveconfiguration will be described with reference to FIGS. 21A and 21B. InFIGS. 21A and 21B, the reference numeral 77 denotes incoming light; 78denotes reflected zero-order diffracted light; 79 denotes an air layerformed by the beams 72 suspended in the air by the isotropic etching ofthe spacer layer 71; 80 denotes an upper reflection film, i.e., thereflection film 73 formed on the beams 72; 81 denotes a lower reflectionfilm, i.e., the reflection film 73 formed on the insulating layer 70;and 82a and 82b denote ± first-order diffracted lights.

FIG. 21A shows the state where no voltage is applied between thenon-oxidized conductive thin film 74 and the substrate 69. Thedifference in the height between the upper reflection film 80 and thelower reflection film 81 is λ/(2 cos θ_(t)), as shown in FIG. 21A. Forexample, if λ is 10 μm and θ_(t) is 25 degrees, then the difference is5.5 μm. In this case, the phase of the light reflected by the upperreflection film 80 is matched with the phase of the light reflected bythe lower reflection film 81. As a result, the diffractive opticalmodulator functions merely as a mirror, and the incoming light 77becomes the zero-order diffracted light 78.

On the other hand, for example, in the state where a positive voltage isapplied between the non-oxidized conductive thin film 74 and thesubstrate 69, the non-oxidized conductive thin film 74 and the substrate69 form a capacitor so as to interpose the air layer 79 and theinsulating layer 70. As a result, the non-oxidized conductive thin film74 functioning as the upper electrode is charged with positive charges,while the substrate 69 functioning as the lower electrode is chargedwith negative charges. Since an electrostatic attracting force is causedbetween these charges, the beams 72 are pulled toward the insulatinglayer 70 until the beams 72 come into contact with the insulating layer70, as shown in FIG. 21B. In this stage, the difference in the heightbetween the surface of the upper reflection film 80 and that of thelower reflection film 81 is 2.8 μm, for example. In this case, the phaseof the light reflected by the upper reflection film 80 is different fromthe phase of the light reflected by the lower reflection film 81 by onehalf of the wavelength (π). As a result, these two lights cancel eachother, so that the zero-order diffracted light disappears and ahigher-order diffracted light other than the zero-order diffracted lightis to be diffracted instead. For example, ± first-order diffractedlights 82a and 82b are generated at the diffraction efficiency of 41%.The light can be modulated by turning on/off the voltage applied betweenthe non-oxidized conductive thin film 74 and the substrate 69.

As described above, in the diffractive optical modulator of thisexample, the spacer layer is formed by an organic film and is removed bythe D/E, so that the beams are not sticked onto the substrate during therinsing and drying processes, unlike a conventional modulator. Inaddition, the spacer layer can be removed uniformly, a plurality ofbeams having the same length are formed, and the variation of theoperation can be eliminated. Moreover, the material for forming thebeams is deposited by a vapor deposition or the like, the residualstress can be controlled easily at a deposition temperature.Furthermore, the residual stress can be suppressed to a small level, adiffractive optical modulator operating at a low voltage can beproduced. In addition, at least the lower surfaces of the beams are madeof a conductive material and a thin oxide film is provided between thelower surfaces of the beams and the substrate, so that the distancebetween the upper and the lower electrodes can be considerably reducedand the modulator can be driven at a low voltage.

Next, a preferable disposition of the respective components of theinfrared sensor of this example for driving the diffractive opticalmodulator at a low voltage will be described. As described above, theoptimal thickness of the beams and the spacer layer of the diffractiveoptical modulator is varied in accordance with the inclination angleθ_(t). FIG. 22A is a graph showing the relationship between theinclination angle and the optimal thickness of the beams and the spacerlayer in the diffractive optical modulator of the infrared sensor ofthis example. FIG. 22B shows the increase in the driving voltage inaccordance with the inclination angle. In FIG. 22B, the driving voltagewhen the inclination angle is 0 degree is standardized as one. As shownin FIG. 22B, if the inclination angle θ_(t) is set to be 45 degrees orless, the increase in the driving voltage can be suppressed to be twiceor less. Also, the present inventors have found if the inclination angleθ_(t) of the diffractive optical modulator is set to be 25 degrees orless, the separation between the incoming light and the zero-orderdiffracted light and a low-voltage driving (the increase in the voltageis suppressed to be 20% or less) can be accomplished simultaneously.More specifically, the diffractive optical modulator, including thebeams having a length of 3 mm and showing the residual stress suppressedto be a tensile stress of +10 MPa or less, can be driven at a lowvoltage of 5 V or less.

Next, referring to FIGS. 23A to 23C, the effects obtained by providing anon-oxidized layer on the lower surfaces of the beams in the diffractiveoptical modulator of this example will be described.

First, the case where the lower surfaces of the beams are not made of aconductive material which is not likely to be oxidized will bedescribed. For example, the lower surfaces of the beams are naturallyoxidized by oxygen in the atmosphere or the lower surfaces are alsooxidized when the spacer layer is removed by the D/E using oxygen plasmaor the like, so that an oxide film is formed. In the diffractive opticalmodulator shown in FIGS. 23A to 23C, the lower surfaces of the beams arenot made of a material which is not likely to be oxidized, unlike thediffractive optical modulator of this example. However, the remainingcomponents of the diffractive optical modulator shown in FIGS. 23A to23C are the same as those of the diffractive optical modulator of thisexample.

FIG. 23A is a cross-sectional view of the diffractive optical modulatorin the longitudinal direction of the beams. FIG. 23B is an enlarged viewof a region in the vicinity of the contact portion, i.e., the region Ashown in FIG. 23A. FIG. 23C is also an enlarged view of the region Awhen the voltage is turned off. In these figures, the reference numeral83 denotes an oxide film which is formed from the beams 72 oxidized byoxygen in the atmosphere or oxygen plasma when ashing is performed; 84denotes the conductive portions of the beams 72 which are not oxidized;85 denotes electrons existing inside or on the surface of the insulatinglayer 70; and 86 denotes holes (or positive charges) formed by themovement of the electrons 85.

FIG. 23A shows the state where a voltage V (>0) is applied between theupper and the lower electrodes. The conductive portion 84 is chargedwith positive charges and the substrate 69 is charged with negativecharges. In FIG. 23B, the electric field is generated by the applicationof a voltage, so that a part of the electrons existing inside or on thesurface of the insulating layer 70 move to the surface or the inside ofthe oxide film 83, thereby forming the holes 84. FIG. 23C shows thestate resulting from the state shown in FIG. 23B by the removal of theapplied voltage. In this state, since no voltage is externally applied,the electrons 85 which have moved to the oxide film 83 do not move tothe insulating film 70, but remain in the oxide film 83, therebygenerating a residual potential difference V_(res). Accordingly, inorder to operate the beams next time, it is necessary to apply a voltagehigher than the previously applied voltage V by the residual potentialdifference V_(res). Therefore, the more times the beams are driven, thehigher the driving voltage becomes. Ultimately, the densities of theelectrons 85 and the holes 86 increase, and the electrostatic forcebetween them also increases, so that the beams 72 remain sticked ontothe insulating layer 70, and can not be restored any longer.

In order to prevent the increase in the driving voltage and the stickingof the beams, it is possible to drive the beams while forcibly movingthe residual charges by the application of a voltage having a waveformshown in FIG. 24. In such a voltage application method, the beams aredriven while removing the residual charges by alternately applyingvoltages having opposite polarities in the regions A and C, therebypreventing the increase of the absolute value of V_(res). However,according to this voltage application method, a voltage source forsupplying a voltage having a twice higher absolute value is requiredduring an actual production. Unless the charges are removed, the numberof the remaining positive or negative charges becomes large; the beamsremain sticked and can no longer be restored.

Next, the effects obtained in the diffractive optical modulator of thisexample where the lower surfaces of the beams are made of a materialwhich is not likely to be oxidized by oxygen in the atmosphere or theoxygen plasma will be described.

FIGS. 25A and 25B illustrate the movement of the charges in the contactportion in the diffractive optical modulator of this example. FIG. 25Ais an enlarged view showing the state of the contact portion when avoltage V (>0) is applied between the substrate 69 and the non-oxidizedconductive thin film 74. In this state, the non-oxidized conductive thinfilm 74 is charged with positive charges, and the substrate 69 ischarged with negative charges. In this stage, the electric field isgenerated by the application of a voltage, so that a part of theelectrons existing inside or on the surface of the insulating layer 70move to the non-oxidized conductive thin film 74, thereby forming theholes 26. However, the electrons 85 which have moved to the non-oxidizedconductive thin film 74 are bonded with the previously charged positivecharges so as to disappear. FIG. 25B shows the state resulting from thestate shown in FIG. 25A by the removal of the applied voltage. Thepositive charges still remain in the insulating layer 70. However, sincethe negative charges do not remain, the electric field is not generatedand the residual potential difference is not generated, either.Accordingly, there is no need for increasing the driving voltage if thebeams are driven many times, and there is no need for forcibly removingthe residual charges by the application of voltages having oppositepolarities. As a result, a low-voltage driving is substantiallyaccomplished and the driving can be performed stably.

As is apparent from the foregoing description, in the infrared sensor ofthis example, by setting the inclination angle θ_(t) in the diffractiveoptical modulator to be 45 degrees or less, the increase in the drivingvoltage can be suppressed to twice or less as compared with the casewhere the light is incoming vertically, i.e., the inclination angleθ_(t) is 0 degrees. Furthermore, if the inclination angle θ_(t) is setto be 25 degrees or less, the length of a beam is set to be 3 mm, andthe residual stress is controlled to be a tensile stress of +10 MPa orless, then a low-voltage driving at 5 V or less becomes possible, acircuit for increasing the voltage becomes unnecessary and the infraredsensor can be produced at a lower cost. In addition, since thepyroelectric element and the signal amplifier are provided on thesupporting plate and the supporting plate is supported by the electrodepins, the infrared sensor can be constructed easily. Moreover, in thediffractive optical modulator for the infrared sensor of this example,the lower surfaces of the beams are made of a conductive material whichis not likely to be oxidized, so that the modulator can be driven byapplying a positive or negative pulse voltage thereto. As a result, thefine control of the waveform of the applied voltage is no longernecessary and the modulator can be driven stably.

In this example, a diffractive optical modulator in which the length ofthe beam is 3 mm is described. Alternatively, by setting the length ofthe beam to be longer, the modulator may be driven at an even lowervoltage with a still larger inclination angle θ_(t). However, if thebeam becomes too long, then the modulator can not operate at a highspeed owing to the effects of the inertial force, and in addition, thebeam is likely to be distorted, thereby degrading the modulationefficiency.

In the diffractive optical modulator of this example, the beam consistsof an elastic body and a non-oxidized conductive thin film.Alternatively, the elastic body may be made of the same material as thatof the non-oxidized conductive thin film. That is to say, the entirebeam may be formed by the non-oxidized conductive thin film. Needless tosay, the reflection film may also be made of the same material. In thisexample, Au is employed as the material which is not likely to beoxidized. Alternatively, Pt, Ti, a NiCr alloy, a CuNi alloy, chromesteel, or other conductive organic materials can also be used. Theelastic body of this example is made of Al. However, the elastic bodycan be made of an insulating material such as an organic material.

In a process for producing the diffractive optical modulator of thisexample, after the elastic body is patterned, the non-oxidizedconductive thin film is patterned by the D/E. Needless to say, thenon-oxidized conductive thin film may be removed by the W/E.Alternatively, the beams may be shaped in the following manner. Afterthe non-oxidized conductive thin film is deposited, the thin film ispatterned once. Thereafter, the elastic body layer is deposited, andthen the elastic body layer is patterned again.

In this example, the deposition is performed mainly by a vapordeposition method. Alternatively, the deposition can be performed byother methods such as a sputtering method or a plating method.

EXAMPLE 7

Hereinafter, an infrared sensor according to a seventh example will bedescribed with reference to FIG. 26. The infrared sensor of this seventhexample is different from the infrared sensor of the sixth example onlyin the positions of the respective components. Thus, the respectivepositions thereof will be described below.

FIG. 26 is a cross-sectional view showing a fundamental configuration ofthe infrared sensor of this example. The inclination angle θ_(t) in thediffractive optical modulator is 25 degrees, as described in the sixthexample. In FIG. 26, the same components as those shown in FIG. 18 aredenoted by the same reference numerals, and the descriptions thereofwill be omitted herein. In FIG. 26, the reference numeral 87 denotes asupporting spacer. The lens 26 is a diffractive lens having an apertureof 3 mm and a focal length of 6 mm, for example, and the spacer 64 hasan inclination angle θ_(t) of 25 degrees. The inclination angle α of thesupporting spacer 87 is set to be 40 degrees so that the infrared lightis vertically incident onto the pyroelectric element 67. In thisexample, since the inclination angle θ_(t) of the spacer 64 is set to be25 degrees, if the length of a beam is set to be 3 mm, driving can beperformed at 0 V and +5 V, for example.

The infrared sensor of this example is provided with a supporting spacer87, thereby vertically receiving the infrared light diffracted by thediffractive optical modulator. As a result, the detection can beconducted at a higher sensitivity. In addition, since the incoming lightcan be easily separated from the zero-order diffracted light diffractedby the diffractive optical modulator, an infrared sensor in a smallersize than the infrared sensor of the sixth example can be produced byusing a lens having a shorter focal length.

EXAMPLE 8

Hereinafter, a diffractive optical modulator according to an eighthexample of the invention will be described with reference to FIGS. 27Aand 27B. FIG. 27A is a plan view showing a diffractive optical modulatorof the eighth example, and FIG. 27B is a cross-sectional view takenalong the line A-A' in FIG. 27A. As shown in FIGS. 27A and 27B, thediffractive optical modulator of this example includes: a substrate 88,e.g., a silicon substrate; an insulating layer 89 formed by an oxidefilm having a thickness of 0.1 μm to be obtained by thermally oxidizingthe silicon substrate 88 or the like; a spacer layer 90 obtained byapplying a photosensitive polyimide film by a spin-coating method,patterning the film by exposing and developing the film, and then bakingthe film; and the beams 91. In this example, the thickness of the spacerlayer 90 and that of the beams 91 are set to be approximately λ/(4 cosθ_(t)). For example, if λ is 10 μm and θ_(t) is 25 degrees, then theoptimal thickness is 2.8 μm. The diffractive optical modulator of thisexample further includes: a reflection film 92 obtained by depositing Au(thickness: 0.1 μm) or the like by a vapor deposition method; anon-oxidized conductive thin film 93 made of a material which is notlikely to be oxidized by oxygen in the atmosphere or oxygen plasma, tobe obtained by depositing Au, Pt or the like by a vapor depositionmethod; and an elastic body 94 obtained by depositing Al or the like bya vapor deposition method. As shown in FIG. 27B, the beam 91 consists ofthe non-oxidized conductive thin film 93 and the elastic body 94.

Hereinafter, referring to FIGS. 28A to 28E and FIGS. 29A to 29E, amethod for producing a diffractive optical modulator of this examplewill be described. FIGS. 28A to 28E are cross-sectional views takenalong the line A-A' in FIG. 27A. FIGS. 29A to 29E are cross-sectionalviews taken along the line B-B' in FIG. 27A.

First, as shown in FIGS. 28A and 29A, a silicon substrate 88 isthermally oxidized at 1050° C. for an hour, for example, thereby formingan insulating layer 89 constituted by the thermally oxidized film havinga thickness of 0.1 μm, for example, on the silicon substrate 88.

Next, as shown in FIGS. 28B and 29B, a photosensitive polyimide film isapplied on the insulating layer 89 by a spin-coating method. After thefilm is exposed and developed so as to form a pattern, the film is bakedat 200° C. for 20 minutes, for example. In this case, the thickness ofthe spacer layer 90 after the baking is set to be 2.8 μm, for example,by adjusting the rotation number of the spinner or the viscosity and thetemperature of polyimide.

Subsequently, as shown in FIGS. 28C and 29C, Au or the like having athickness of 0.1 μm, for example, is deposited on the spacer layer 90,thereby forming the non-oxidized conductive thin film 93, and then an Alfilm or the like having a thickness of 2.7 μm, for example, is furtherdeposited thereon by a vapor deposition, thereby forming the elasticbody 94. In this case, by setting the gap (indicated by δ in FIG. 27B)of the polyimide pattern to be less than twice of the sum of thethickness of the non-oxidized conductive thin film 93 and that of theelastic body 94, it is possible to fill the gap of the polyimide patternas shown in FIG. 28C. In this example, δ is set to be 3 μm.

Then, as shown in FIGS. 28D and 29D, a positive type photoresist isapplied onto the elastic body 94, and exposed and developed so as toform a mask 102. Thereafter, Al forming the elastic body 94 is removedby a W/E using an etching solution composed of phosphoric acid, aceticacid and nitric acid, for example. The non-oxidized conductive thin film93 is then etched by the D/E.

Next, as shown in FIGS. 28E and 29E, the mask 95 is removed by the D/Eusing the oxygen plasma or the like, and at the same time, the spacerlayer 90 including the portions under the beams 91 is isotropiclyetched. Since some portions of the spacer layer 90 are not in contactwith oxygen plasma, the portions are not removed but remain. Finally, Auor the like having a thickness of 0.1 μm is deposited by a vapordeposition method, thereby forming the reflection film 92. By performingthe above steps, a diffractive optical modulator having a configurationas shown in FIGS. 27A and 27B is completed.

The diffractive optical modulator having such a configuration operatesin a similar manner to the diffractive optical modulator of the sixthexample. Therefore, by turning on/off the voltage applied between thesubstrate 88 and the non-oxidized conductive thin film 93, the incominglight can be modulated.

In the diffractive optical modulator of the sixth example, the length ofa beam is determined by the time required for removing the spacer layer.On the other hand, in the diffractive optical modulator of this example,since the length of a beam is determined by the pattern of the spacerlayer, a diffractive optical modulator including the beams of the samelength can be produced with satisfactory reproducibility. Since thelength of the beam affects the driving voltage of a diffractive opticalmodulator, the method for producing a diffractive optical modulator ofthis example eliminates the variation of the driving voltage from thediffractive optical modulator.

EXAMPLE 9

Hereinafter, an infrared sensor according to a ninth example of theinvention will be described with reference to FIG. 30.

The infrared sensor of this ninth example can be suitably used in thecase where the signal/noise (S/N) ratio is desired to be large indetecting the infrared light, and in the case where an infrared sensorof an extremely small size is produced. Generally, in the case where theS/N ratio is set to be large in detecting the infrared light, by turningon/off the voltage applied to the diffractive optical modulator, anelectromagnetic noise adversely affecting the pyroelectric element andthe signal amplifier is generated, so that the noise becomes large. Onthe other hand, in the case of producing a small-sized infrared sensor,the size of the diffractive optical modulator included therein alsobecomes small. As a result, since the length of a beam becomes short,the modulator can not operate unless a relatively high voltage isapplied thereto. If such a high voltage is applied, a largeelectromagnetic noise is also generated, so that the pyroelectricelement and the signal amplifier experience strong noise interference.

FIG. 30 is a side view showing a fundamental configuration of theinfrared sensor of this example. In FIG. 30, the same components asthose shown in FIG. 18 will be denoted by the same reference numerals,and the description will be omitted herein. In FIG. 30, anelectromagnetic shield 96 is grounded by lines (not shown), and is madeof a conductive material transmitting the infrared light. Since anantireflection film is provided on the surface of the electromagneticshield 96, the zero-order diffracted light diffracted by the diffractiveoptical modulator is transmitted through the electromagnetic shield 96with substantially no loss. On the other hand, the electromagnetic noisegenerated when the diffractive optical modulator is driven is shieldedby the electromagnetic shield 96, and therefore, the noise no longeradversely affects the pyroelectric element 67 and the signal amplifier68.

As is apparent from the foregoing description, in the infrared sensor ofthis example, a shield is provided between the diffractive opticalmodulator and a device such as a pyroelectric element or a signalamplifier, and is grounded, whereby the effects of the electromagneticnoise generated from the diffractive optical modulator can be shielded.

EXAMPLE 10

Hereinafter, an infrared sensor according to a tenth example of theinvention will be described with reference to FIG. 31. The infraredsensor of this tenth example is different from the infrared sensor ofthe sixth example only in the configuration of the diffractive opticalmodulator. The diffractive optical modulator of this example isconstructed so that an electromagnetic noise is not generated at leasttoward the portion in which the pyroelectric element and the signalamplifier are disposed. Therefore, the electromagnetic shield asdescribed in the ninth example is not required to be provided.

The diffractive optical modulator of this example will be describedbelow. As shown in FIG. 31, the diffractive optical modulator of thisexample includes: a substrate 97, e.g., a silicon substrate, whose bothsurfaces are mirror-polished; an antireflection film 98 made of aninsulating material, e.g., a ZnS film having a thickness of 1.1 μm; areflection film 99 obtained by depositing Au (thickness: 0.1 μm) or thelike and patterning it; and a spacer layer 100 obtained by applying apolyimide film or the like by a spin-coating method, and baking thefilm. In this example, the optimal thickness of the spacer layer 100 isapproximately λ/(4 cos θ_(t)). For example, if λ is 10 μm and θ_(t) is25 degrees, then the optimal thickness is 2.8 μm. The diffractiveoptical modulator of this example further includes: a non-oxidizedconductive reflection film 101 made of a conductive material which isnot likely to be oxidized by oxygen in the atmosphere or oxygen plasmaand has substantially the same reflectance as that of the reflectionfilm 99, to be obtained by depositing Au (thickness: 0.1 μm) or the likeby a vapor deposition method; and an elastic body 102 obtained bydepositing Al (thickness: 1 μm) or the like by a vapor depositionmethod.

Referring to FIGS. 32A to 32F, a method for producing a diffractiveoptical modulator of this example will be described. In these figures,the reference numerals 103 and 104 denote masks; and 105 denotes a beamconsisting of the non-oxidized conductive reflection film 101 and theelastic body 102. In these figures, the same components as those in FIG.31 will be denoted by the same reference numerals and the descriptionthereof will be omitted herein.

First, as shown in FIG. 32A, ZnS films having a thickness λ/(4 n) (n isthe refractive index of ZnS=2.3) of 1.1 μm are deposited on both of themirror-polished surfaces of the silicon substrate 97 by a sputteringmethod or the like, thereby forming the antireflection film 38.

Next, as shown in FIG. 32B, Au (thickness: 0.1 μm) or the like isdeposited by a vapor deposition method; a positive photoresist or thelike is applied thereon by a spin-coating method and exposed anddeveloped so as to form the mask 103; and then the deposited Au isremoved by the W/E using an etching solution composed of iodine,potassium iodide and the like, thereby forming the reflection film 99.

Then, as shown in FIGS. 32C, the mask 103 is removed and a polyimidefilm or the like is applied thereon by a spin-coating method so as toform the spacer layer 100. After the polyimide film is applied, the filmis baked at 200° C. for 20 minutes, for example. In this case, thethickness of the spacer layer 100 after the baking is set to be λ/(4 cosθ_(t)) and the unevenness of the reflection film is smoothed, byadjusting the rotation number of the spinner or the viscosity and thetemperature of polyimide.

Subsequently, as shown in FIG. 32D, Au (thickness: 0.1 μm) or the likeis deposited on the spacer layer 100 by a vapor deposition method or thelike so as to form the non-oxidized conductive reflection film 101, andan Al film or the like having a thickness of 1 μm, for example, isdeposited thereon by a vapor deposition method or the like so as to formthe elastic body 102. Thereafter, a positive type photoresist or thelike is applied thereon, and then exposed and developed so as to formthe mask 104.

Next, as shown in FIG. 32E, Al forming the elastic body 102 is removedby the W/E using an etching solution composed of phosphoric acid, aceticacid and nitric acid, for example, and the non-oxidized conductivereflection film 101 is etched by the D/E using chlorine or the like.

Finally, as shown in FIG. 32F, the mask 104 is removed by the D/E usingoxygen plasma or the like, and at the same time, the spacer layer 100including the portions under the beams 105 is isotropicly etched. Byperforming the above-described steps, the diffractive optical modulatorhaving a configuration shown in FIG. 30 is completed.

Referring to FIGS. 33A and 33B, the operation of the diffractive opticalmodulator of this example will be described. In these figures, thereference numeral 106 denotes incoming light; 107a and 107b denote ±first-order diffracted lights; 108 denotes an air layer formed by thebeams 52 suspended in the air by the isotropic etching of the spacerlayer 100; and 49 denotes reflected light. FIG. 33A shows the statewhere no voltage is applied between the non-oxidized conductivereflection film 101 and the substrate 97. The difference in the heightbetween the reflection film 99 and the non-oxidized conductivereflection film 101 is λ/(4 cos θ_(t)) as shown in FIG. 33A. Forexample, if λ is 10 μm and θ_(t) is 25 degrees, then the difference is2.8 μm. The incoming light 106 passes through the upper antireflectionfilm 98, the silicon substrate 97, and then the lower antireflectionfilm 98, so as to be incident onto the grating portion consisting of thereflection film 99 and the non-oxidized conductive reflection film 101.In this case, since the phase of the light reflected by the reflectionfilm 99 is different from the phase of the light reflected by thenon-oxidized conductive reflection film 101 by one half of thewavelength π, these two light cancel each other and a diffracted lighton a higher order is diffracted. For example, the ± first-orderdiffracted lights 107a and 107b are generated at a diffractionefficiency of 41%, respectively.

FIG. 33B shows the state where a positive voltage is applied between thenon-oxidized conductive reflection film 101 and the substrate 97, forexample. The substrate 97 is grounded. In this case, the non-oxidizedconductive reflection film 101 and the substrate 97 form a capacitor soas to interpose the air layer 108 and the antireflection film 98. Thenon-oxidized conductive reflection film 101 functioning as the lowerelectrode is charged with positive charges, while the substrate 97functioning as the upper electrode is charged with negative charges.Since an electrostatic attracting force is caused between these charges,the beams 105 are pulled towards the antireflection film 98 until thebeams 105 come into contact with the film 98, as shown in FIG. 33B. Inthis case, the surface of the reflection film 99 and the surface of thenon-oxidized conductive reflection film 101 are on the same plane, sothat the diffractive optical modulator functions as a mirror, and allthe incoming light 106 becomes the reflected light 109.

As is apparent from the foregoing description, by turning on/off thevoltage applied between the non-oxidized conductive reflection film 101and the substrate 97, the light can be modulated.

In the diffractive optical modulator for the infrared sensor of thisexample, the substrate 97 is grounded, a voltage is applied to thenon-oxidized conductive reflection film 101 and the incoming light isdirected through the grounded substrate 97 to the reflection film.Therefore, in detecting very weak infrared light, or in driving thediffractive optical modulator by applying a relatively high voltage, anelectromagnetic noise is not generated on the light-modulation sidebecause the substrate 97 itself functions as the electromagnetic shield.

In the diffractive optical modulator of this example, the antireflectionfilm functions as the insulating layer and the film has a largethickness, e.g., a ZnS film having a thickness of 1.1 μm. Thus thevoltage required for driving the modulator seems to be high. However,since the actual relative dielectric constant of ZnS is 8 or more, theeffective length, i.e., an eighth of the thickness, is 0.14 μm (=1.1μm÷8). Accordingly, the driving voltage does not become too high. Inaddition, since the thickness of a beam 105 is not particularly limitedby the wavelength λ or the inclination angle θ_(t), the diffractiveoptical modulator of this example can be driven at a lower voltage ascompared with the diffractive optical modulator of the first or thethird example, by setting the thickness of the beam to be smaller. Morespecifically, if the wavelength λ is 10 μm, the inclination angle θ_(t)is 25 degrees and the thickness of the beam is set to be 2 μm or less,then the modulator of this example can be driven at 0, 5 V.

EXAMPLE 11

Hereinafter, an infrared sensor according to an eleventh example of theinvention will be described with reference to FIG. 34. The infraredsensor of this eleventh example is different from the infrared sensorsof the foregoing examples in that the lens for converging the infraredlight on the pyroelectric element is disposed between the pyroelectricelement and the diffractive optical modulator. In the foregoingexamples, the lens for converging the infrared light is disposed so asto cover the opening of the seal case.

As shown in FIG. 34, the infrared sensor of this example includes: adiffractive optical modulator 209 for diffracting at least a part of theincoming infrared light 206 as the zero-order diffracted light 211; adiffractive lens 208; and a pyroelectric element 202. The diffractivelens 208 converges the infrared light diffracted by the diffractiveoptical modulator 209 onto the pyroelectric element 202. A diffractiveoptical modulator of any of the foregoing examples can be used as thediffractive optical modulator 209.

In this example, the diffractive optical modulator 209, the pyroelectricelement 202 and the lens 208 are included in the seal case 205 having anopening. That is to say, the opening of the seal case 205 is notcovered. The diffractive lens 208 is a lens obtained by forming adiffraction grating on a substrate functioning as an incoming infraredwavelength filter 204. In this example, the diffractive lens 208 itselfoperates as a part of seal case 205.

In this example, since approximately collimated light is incident ontothe diffractive optical modulator 209, the modulation efficiency isimproved. In addition, the noise generated by the operation of thediffractive optical modulator 209 is not likely to affect thepyroelectric element 202.

EXAMPLE 12

Hereinafter, an infrared sensor according to a twelfth example of theinvention will be described with reference to FIG. 35. The infraredsensor of this twelfth example is different from the infrared sensors ofthe foregoing Examples 1 to 10 in that the lens for converging theinfrared light on the pyroelectric element is disposed between thepyroelectric element and the diffractive optical modulator. In Examples1 to 10, the lens for converging the infrared light is disposed so as tocover the opening of the seal case.

The infrared sensor of this twelfth example is different from theinfrared sensor of the eleventh example shown in FIG. 34 in that theopening of the seal case 205 is covered with the incoming infraredwavelength filter 204. A lens 208' of this example is not a diffractivelens, but a polished lens made of silicon, or a polyethylene lens. Inthis example, only the light transmitted through the incoming infraredwavelength filter 204 is incident onto the diffractive optical modulator209. Since the diffractive optical modulator 209 is disposed within asealed environment, the resistivity of the diffractive optical modulator209 against the environment is improved.

EXAMPLE 13

Hereinafter, an infrared sensor according to a thirteenth example of theinvention will be described with reference to FIG. 36. The infraredsensor of this thirteenth example is different from the infrared sensorof the eleventh example shown in FIG. 34 in that a cylindrical openingcontrol member 215 is provided on the opening of the seal case 205, asis apparent from FIG. 36. The aperture of this opening control member215 is set to be 3 mm, and the length thereof in the axial direction isset to be 30 mm, for example. The opening control member 215 is made ofa material cutting off the infrared light. The opening control member215 prevents the infrared light other than the infrared light outputfrom the object to be detected from being incident onto the diffractiveoptical modulator, thereby improving the S/N ratio of the output signal.

In the foregoing Examples 1 to 13, a diffractive optical modulator ofthe invention is applied to an infrared sensor. In the followingexamples, a display device utilizing a diffractive optical modulator ofthe invention will be described.

EXAMPLE 14

First, referring to FIG. 37, a fundamental arrangement of a displaydevice according to an example of the invention will be described. Inthis example, the light emitted from a light source 224 is diffracted bya diffractive optical modulator 225, and then converged by a projectionlens 226. The intensity of the light output from the diffractive opticalmodulator 225 to the lens 226 is modulated in accordance with thevoltage applied to the diffractive optical modulator 225.

Next, referring to FIG. 38, a more detailed arrangement of the displaydevice according to this example of the invention will be described. Thedisplay device shown in FIG. 38 includes: a white light source 224 suchas a metal halide lamp and a xenon lamp; a dichroic mirror 229a forselectively reflecting red light; a dichroic mirror 229b for selectivelyreflecting blue light; and a dichroic mirror 229c for selectivelyreflecting green light. A cold mirror 227 for transmitting a heat ray orinfrared ray and for reflecting visible light is disposed behind thelight source 224. The light emitted from the light source 224 as well asthe light reflected by the cold mirror 227 is collimated or converged bya converging lens 228.

In this example, the three diffractive optical modulators 225a to 225chaving the above-described configuration are used. The diffractiveoptical modulator 225a is disposed at a position so as to receive thered (R) light reflected by the dichroic mirror 229a. The diffractiveoptical modulator 225b is disposed at a position so as to receive theblue (B) light reflected by the dichroic mirror 229b. The diffractiveoptical modulator 225c is disposed at a position so as to receive thegreen (G) light transmitted by the dichroic mirror 229b. In thisexample, the red light diffracted by the diffractive optical modulator225a is directed to a coupling prism 231 by a mirror 230a. The greenlight diffracted by the diffractive optical modulator 225c issuccessively reflected by the dichroic mirror 229c and the mirror 230bso as to be directed to the coupling prism 231. The blue lightdiffracted by the diffractive optical modulator 225b is transmittedthrough the dichroic mirror 229c, and then reflected by the mirror 230b,so as to be directed to the coupling prism 231. The light output fromthe coupling prism 231 is directed through the projection lens 226 so asto form an image on a screen (not shown).

The three diffractive optical modulators 225a to 225c are switched on apixel basis by a controller (not shown). As a result, the incoming lightis modulated on a pixel basis, and the spatial modulation of theincoming light is accomplished by the pixels output by the zero-orderdiffracted light and the pixels not output by the zero-order diffractedlight.

If the ratio of a lattice pitch Λ of the diffractive optical modulatorto the central wavelength λ of the incoming light is seven, then thediffraction angle of the first-order diffracted light generated by thediffractive optical modulator becomes 8.2 degrees. Accordingly, in orderto prevent the first-order diffracted light from entering the apertureof the projection lens 226, the F value (=focal length/effectiveaperture of lens) of the projection lens is required to be 3.5 or more.Therefore, the F value of the converging lens is also required to be 3.5or more. By using such a lens, the light amount projected onto thescreen becomes substantially uniform.

In this example, the light emitted from the light source is separatedinto the three primary colors of red, green and blue, three diffractiveoptical modulators are provided so as to correspond to the respectivecolors, and λ/Λ is set to be seven, whereby a display device with a highoptical efficiency for projecting substantially uniform amount of lightonto the screen can be obtained.

EXAMPLE 15

Referring to FIGS. 39A and 39B, a display device according to anotherexample of the invention will be described. The display device of thisexample is different from the display device of the fourteenth exampleonly in the diffractive optical modulator. Therefore, the componentsother than the diffractive optical modulator will not be describedherein.

FIGS. 39A and 39B show the configuration of one pixel of the diffractiveoptical modulator. In these figures, the reference numerals 232a to 232edenote beams having a thickness of one quarter of the wavelength of theincoming light. Aluminum, silver or the like is deposited on the uppersurfaces of the beams 232a to 232e so as to function as the electrodeand the reflection film. A spacer 233 also has a thickness of onequarter of the wavelength of the incoming light. An electrode 234,provided on a substrate (not shown), reflects the incoming light. Theelectrode 234, which is temporarily disposed below the spacer 233 forvisual convenience, is in the same plane of the lower surface of thespacer 233 as shown in FIG. 39B.

In the diffractive optical modulator of this example, the beams 232a and232e provided on the spacer 233 are not deflected even upon theapplication of the electrostatic force. Therefore, the differencebetween the phase of the light reflected by the upper surfaces of thebeams 232a and 232e and the phase of the light reflected by the uppersurface of the spacer 233 is always one half of the wavelength. Also, inthe portions of the beams 232b, 232c and 232d supported by the spacer233, the phase difference is always one half of the wavelength.Accordingly, the phase difference is one half of the wavelength in allthe regions between adjacent pixels. Consequently, since the zero-orderdiffracted light is not generated from a region between adjacent pixels,the respective pixels can be separated easily without particularlyproviding a black matrix.

EXAMPLE 16

Referring to FIGS. 40A and 40B, a display device according to stillanother example of the invention will be described.

The display device of this example is different from the display deviceof the fourteenth example only in the diffractive optical modulator.Therefore, the components other than the diffractive optical modulatorwill not be described herein. In these figures, a dielectric film 235has a thickness of one quarter of the wavelength of the incoming light,and aluminum, silver or the like is deposited on the upper surface ofthe dielectric film 235 so as to function as the electrode and thereflection film. Slits 236a to 236l are produced by etching thedielectric film 235. A supporting beam 237 is also produced by etchingthe dielectric film 235. A spacer is denoted by 238, and an electrode239 also reflects the light.

Next, the operation of the diffractive optical modulator of this examplewill be described. The dielectric film 235 is supported in the air bythe supporting beam 237 and the spacer 238. When a voltage is appliedbetween the electrode provided on the upper surface of the dielectricfilm 235 and the electrode 239, an electrostatic force is generatedtherebetween, so that the dielectric film 235 comes into contact withthe electrode 239. Since the width of the supporting beam 237 is smallerthan the width of the movable portion of the dielectric film 235, thesupporting beam 237 is likely to be deformed, and the area of thecontact portion between the dielectric film 235 and the electrode 239becomes much large. Consequently, the area for modulating the zero-orderdiffracted light becomes large, and therefore, effective aperturebecomes.

By making the width of the supporting beam smaller than the width of themovable portion, the supporting beam is likely to be deformed. As aresult, the dead space not contributing to the modulation of thezero-order diffracted light can be advantageously reduced.

In this example, the same effects can be attained by making the width ofeach of the beams constituting the diffractive optical modulator smalleronly in the vicinity of the spacer 238.

In the diffractive optical modulator of the invention, an insulatinglayer is provided between a movable grating and a plate, so that thematerial for constituting the grating can be freely selectedirrespective of the conductivity thereof. If the beam is made of aconductive material, or if the lower surface of the beam is made of aconductive material so as to function as a second electrode, then thegap between the plate and the second electrode can be reduced, therebyenabling the operation at a low voltage. In the case of using thediffractive optical modulator for infrared light having a relativelylong wavelength, the reduction of the distance between the electrodes iseffective.

In addition, if the lower surface of the beam is made of a conductivematerial which is not likely to be oxidized, no residual charges remainin the contact portion when the modulator is driven, so that theresidual potential difference is not generated. Accordingly, there is noneed for increasing the driving voltage as the modulator is driven manytimes, and there is no need for forcibly removing the residual chargesby the application of the voltages having opposite polarities.Therefore, a power supply for applying voltages having oppositepolarities is no longer necessary, thereby realizing a lower cost. Inaddition, the fine control of the voltage waveform also becomesunnecessary, and the modulator can always be driven stably.

According to a method for producing a diffractive optical modulator ofthe invention, a spacer layer is formed by an organic film and apredetermined portion thereof is removed by a dry etching process,whereby the sticking of the beams onto the substrate during the rinsingand the drying processes, which has conventionally been generated, canbe prevented. In addition, since the spacer layer is uniformly etched,beams having the same length can be formed; thereby, the variation ofthe operation of the modulator can be eliminated.

Since the length of a beam is determined by the pattern of a spacerlayer, a diffractive optical modulator including the beams having thesame length can be produced with satisfactory reproducibility. Also, theminimum driving voltage of the diffractive optical modulator isdetermined by the length of the beam, and therefore, the drivingvoltages among a plurality of diffractive optical modulators can bealigned.

Furthermore, according to the invention, a mechanical chopper is nolonger necessary, and it is possible to provide a downsized infraredsensor operating at a lower power consumption. The durability thereof isalso improved. If the inclination angle θ_(t) of the diffractive opticalmodulator is set to be 45 degrees or less, for example, the increase inthe driving voltage can be suppressed to twice or less as compared withthe case where the light is vertically incident. Further, if theinclination angle θ_(t) is set to be 25 degrees or less, the length ofthe beam is set to be 3 mm or less and the residual stress is controlledto be a tensile stress of +10 MPa or less, then the modulator can bedriven at 5 V or less. As a result, a circuit for increasing the voltageis no longer necessary, and a lower cost is realized. Moreover, byproviding the pyroelectric element and the signal amplifier on thesupporting plate and by supporting the supporting plate by the electrodepins, the infrared sensor can be constructed easily.

The display device of the invention realizes the spatial modulation ofthe light at a larger effective aperture, as compared with a liquidcrystal display device. In addition, since the polarization is no longernecessary, the optical efficiency is improved. Therefore, a brighterimage can be obtained.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A diffractive optical modulator comprising:aplate having a portion functioning as a first electrode; a spacer layer,formed on the plate; and a grating consisting of a plurality of beamshaving a portion functioning as a second electrode, both ends of thebeams being supported on the spacer layer; wherein, by adjusting avoltage applied between the first electrode and the second electrode, adistance between the beams and the plate is varied, thereby controllingthe diffraction efficiency, wherein an insulating layer is furtherprovided between the plate and the plurality of beams, and wherein athickness and a vertically movable distance of each of the plurality ofbeams depends on θ_(t), where θ_(t) refers to an incident angle of lightmeasured with respect to a plane which is perpendicular to a top surfaceof the plurality of beams and which is parallel to a longitudinal extentof the plurality of beams.
 2. A diffractive optical modulator accordingto claim 1, wherein a reflection film is formed on a surface of theinsulating layer and on surfaces of the beams.
 3. A diffractive opticalmodulator according to claim 1, wherein the plate is made of asemiconductor functioning as the first electrode.
 4. A diffractiveoptical modulator according to claim 1, wherein the plate consists of aconductive layer functioning as the first electrode and an insulatingsubstrate for supporting the conductive layer.
 5. A diffractive opticalmodulator according to claim 1, wherein at least lower surfaces of thebeams are made of a conductive material.
 6. A diffractive opticalmodulator according to claim 1, wherein at least lower surfaces of thebeams are made of a conductive material which resists oxidation.
 7. Adiffractive optical modulator according to claim 1, wherein the spacerlayer is made of an organic material.
 8. A diffractive optical modulatoraccording to claim 5, wherein the conductive material is selected fromthe group consisting of Au, Pt, Ti, an NiCr alloy, a CuNi alloy, chromesteel, and a conductive organic material.
 9. A diffractive opticalmodulator according to claim 1, wherein the spacer layer is made of thesame material as a material of the plurality of beams.
 10. A diffractiveoptical modulator according to claim 9, wherein a width of the beamssupported on the spacer layer in a longitudinal direction is less thantwice of a thickness of the beams.
 11. A diffractive optical modulatorcomprising:a plate having a portion functioning as a first electrode,and an upper surface and a bottom surface; a spacer layer formed on theupper surface of plate; and a grating consisting of a plurality of beamshaving a portion functioning as a second electrode, both ends of thebeams being supported on the spacer layer; wherein, by adjusting avoltage applied between the first electrode and the second electrode, adistance between the beams and the plate is varied, thereby controllingthe diffraction efficiency, and wherein a first antireflection film madeof an insulating material is further provided on the upper surface ofthe plate, and a second antireflection film made of an insulatingmaterial is further provided on the bottom surface of the plate, andwherein each of the beams consists of a beam-shaped reflection filmfunctioning as the second electrode and being made of a conductivematerial, and an elastic layer formed on the beam-shaped reflectionfilm.
 12. A diffractive optical modulator comprising:a plate having aportion functioning as a first electrode; a spacer layer formed on theplate; and a grating consisting of a plurality of beams having a portionfunctioning as a second electrode, both ends of the beams beingsupported on the spacer layer; wherein, by adjusting a voltage appliedbetween the first electrode and the second electrode, a distance betweenthe beams and the plate is varied, thereby controlling the diffractionefficiency, wherein the plurality of beams are arranged so that amovable distance between the plurality of beams and the plate becomesminimum on an optical axis of incoming light, and wherein a thicknessand a vertically movable distance of each of the plurality of beamsdepends on θ_(t), where θ_(t) refers to an incident angle of lightmeasured with respect to a plane which is perpendicular to a top surfaceof the plurality of beams and which is parallel to a longitudinal extentof the plurality of beams.
 13. A diffractive optical modulatorcomprising:a plate having a portion functioning as a first electrode; aspacer layer formed on the plate; and a grating consisting of aplurality of beams having a portion functioning as a second electrode,both ends of the beams being supported on the spacer layer; wherein, byadjusting a voltage applied between the first electrode and the secondelectrode, a distance between the beams and the plate is varied, therebycontrolling the diffraction efficiency, wherein a thickness of theplurality of beams is adjusted so as to be minimal on an optical axis ofincoming light, and wherein a thickness and a vertically movabledistance of each of the plurality of beams depends on θ_(t), where θ_(t)refers to an incident angle of light measured with respect to a planewhich is perpendicular to a top surface of the plurality of beams andwhich is parallel to a longitudinal extent of the plurality of beams.14. A diffractive optical modulator according to claim 11, wherein thespacer layer is made of an organic material.
 15. A diffractive opticalmodulator according to claim 11, wherein the first electrode isgrounded, and a voltage is applied to the beam-shaped reflection film.16. A diffractive optical modulator according to claim 11, wherein theelastic layer is made of the same material as a material of thebeam-shaped reflection film.
 17. A diffractive optical modulatoraccording to claim 11, wherein the conductive material is selected fromthe group consisting of Au, Pt, Ti, an NiCr alloy, a CuNi alloy, chromesteel, and a conductive organic material.
 18. A method for producing adiffractive optical modulator of claim 13, comprising the stepsof:depositing a first layer functioning as a spacer layer on a plate;and depositing a second layer functioning as beams on the spacer layer,wherein, during the step of depositing the first layer, the first layeris deposited while moving a shield disposed between a deposition sourcefor supplying a material of the first layer towards the plate and theplate, thereby varying a thickness of the first layer at respectivepositions.
 19. A method for producing a diffractive optical modulator ofclaim 13, comprising the steps of:depositing a first layer functioningas a spacer layer on a plate; and depositing a second layer functioningas beams on the spacer layer, wherein, during the step of depositing thesecond layer, the second layer is deposited while moving a shielddisposed between a deposition source for supplying a material of thesecond layer towards the plate and the plate, thereby varying athickness of the second layer at respective positions.
 20. A method forproducing a diffractive optical modulator of claim 7, comprising thesteps of:forming an insulating film on a plate having a portionfunctioning as a first electrode; depositing an organic film on theinsulating film; depositing a conductive thin film on the organic film;patterning the conductive thin film, thereby forming a plurality ofbeams functioning as a second electrode; and removing a predeterminedportion of the organic film by a dry etching process, thereby forming aspacer for supporting both ends of the plurality of beams.
 21. A methodfor driving a diffractive optical modulator of claim 5, wherein voltagesin a rectangular waveform having an equal absolute value and oppositepolarities are applied to the first electrode and the second electrode,respectively.
 22. A method for driving a diffractive optical modulatorof claim 11, wherein voltages in a rectangular waveform having an equalabsolute value and opposite polarities are applied to the firstelectrode and the second electrode, respectively.
 23. A display devicecomprising:a light source; a diffractive optical modulation unitprovided on an optical path of light emitted from the light source; andan optical element for imaging light output from the diffractive opticalmodulation unit, wherein the diffractive optical modulation unit isprovided with a diffractive grating means, thereby controlling azero-order diffraction efficiency of the diffractive grating means, andwherein the diffractive grating means comprises a plurality of beams anda thickness and a vertically movable distance of each of the pluralityof beams depends on θ_(t), where θ_(t) refers to an incident angle oflight measured with respect to a plane which is perpendicular to a topsurface of the plurality of beams and which is parallel to alongitudinal extent of the plurality of beams.
 24. A display deviceaccording to claim 23, wherein the diffractive grating means is areflective type means.
 25. A display device according to claim 23,wherein a lattice pitch of the diffractive grating means is seven timesor more of a central value of a waveband of the light.
 26. A displaydevice according to claim 23, wherein the diffractive optical modulationunit comprises a plurality of diffractive optical modulatorstwo-dimensionally arranged as the diffractive grating means, and theplurality of diffractive optical modulators respectively correspond to aplurality of pixels,each of the plurality of diffractive opticalmodulators comprising: a plate having a portion functioning as a firstelectrode; a spacer layer formed on the plate; and a grating consistingof a plurality of beams having a portion functioning as a secondelectrode, both ends of the beams being supported on the spacer layer;the diffractive optical modulator controlling the diffraction efficiencyby varying a gap between the beams and the plate by adjusting a voltageapplied between the first electrode and the second electrode.
 27. Adisplay device according to claim 26, wherein the plurality ofdiffractive optical modulators further comprise an insulating layerformed between the plate and the plurality of beams.
 28. A displaydevice according to claim 26, wherein a region for forming a phasedifference which is one half of a wavelength of the light is providedbetween adjacent modulators of the plurality of diffractive opticalmodulators.
 29. A display device according to claim 23, furthercomprising a separation means for separating the light emitted from thelight source into a plurality of light beams having differentwavebands,wherein the diffractive optical modulation unit is disposed onan optical path of each of the plurality of light beams.
 30. A displaydevice according to claim 23, wherein the diffractive optical modulationunit comprises a plurality of diffractive optical modulatorstwo-dimensionally arranged as the diffractive grating means, and theplurality of diffractive optical modulators respectively correspond to aplurality of pixels,each of the plurality of diffractive opticalmodulators comprising: a plate having a portion functioning as a firstelectrode; a supporting beam formed on the plate; and a gratingconsisting of a plurality of beams having a portion functioning as asecond electrode, both ends of the beams being supported on thesupporting beam; a width of the supporting beam being smaller than awidth of a movable portion of each of the plurality of beams, thediffractive optical modulator controlling a diffraction efficiency byvarying a gap between the beams and the plate by adjusting a voltageapplied between the first electrode and the second electrode.